Ask your players what kind of game they want.

Players love getting away with stuff and living vicarious lives where they can go crazy things and get away with them. That's part of the fun of playing.

But, they also know that the game isn't fun if it devolves into total chaos and random orphanage burning. So, they do want a world which responds to their actions. They want actions which have rewards and consequences. They want to avoid consequences which take away things they find fun.

Make them take responsibility.

Your players are co-authors of this world with you. Whatever in-world, or game-mechanic, rules you give them will either be interpreted as systems to overcome, or impediments on their fun, of what they really want is to be madcap.

So, explain things to them and ask how they want the world to work.

Do they want to play in a grand theft auto world where they can get away with anything... But life is cheap and they could lose their characters? Something more realistic, even gritty? Do they want to be shining heroes, in a world with dastardly villains?

Once you get them to say what they want, as players, then hold them to that. If they say they want to be heroes and they don't play that way, then break the scene and ask the table if this is the direction they really want to go. If the players disagree, take a vote. The players on the losing side of that vote can decide to alter their character interpretation, or play new characters.

You get a say, too.

Whenever I start a new game, I lay down ground rules. I'm here for my enjoyment, too, and some things aren't fun for me. PVP combat isn't fun for me, if player conflict occurs, it's resolved by a metagame discussion about what way people want the story to go. People playing evil characters isn't fun for me, therefore characters being created should be not fundamentally inclined toward such things, and if they evolve in that direction, they become NPCs.

You have the right to say "I'm not interested in running grand theft auto, fantasy edition. That's not fun for me. If those are the kinds of decisions your character is going to make, then they don't fit the genre of the story we're telling."

You can't force somebody to play the way you want to. You can ask them to make a decision endorsing a particular understanding of genre and consequences, but, in the end, no matter what you say, or what you do, if a player wants to play a different kind of game than you do... Then they should play that game with other people who want to play that game with them. It's an unfortunate fact of life, but I think every gaming group ends up confronting that eventually.

I really like @KorvinStarmast's answer, but I want to add another option.

Even if the players manage to get safely away after robbing the Captain, you could determine that the Captain now cannot proceed with helping them in turn, without having his purse back. He can be a vital part of some quest where the players need his help, but he cannot help after having a huge sum of money stolen (which did not belong to him, so he has to sell his magical sword to return the debt, for example).

Play a mental game with the thief. In the inn where they stayed one morning, he hears though the door the sound of heavily armored people stepping into the corridor. They shout, then knock down a door - not his door, but his neighbor's. The poor guy goes with them, for ... well, some crime, that he shouts he did not commit. Maybe the crime was not related to this case, maybe it was. Worst-case scenario, a guy can die, and have his ghost find the real thief to haunt him.

One DM I had made a habit of equipping NPCs with purses that had been enchanted with Magic Mouth to scream for help whenever someone other than the owner tries to remove them. I would consider having a few people of importance (who have access to a friendly mage) use these. Then a brief conversation with the unsuccessful thief ("You know, we usually chop off your hand for a first offense, but...") and a flogging (~half hitpoints, half movement for a few days, local clerics not willing to heal you).

For a good read, look up a copy of Joel Rosenberg's The Sleeping Dragon. He has a character make the mistake of picking the wrong pocket (and die immediately thereafter).

I think the way you word the question shows where possibly you are running into a problem here.

You ask how "I" as the DM can impose consequences. How do you punish or reward your players? But that is not actually your job, and certainly not the way I run my campaigns.

The question I ask is:

How does the world and the characters in it react to what has just happened?

I am running a world, with people. I have some modeled in detail such as named and major NPCs, others modeled vaguely such as the weaponsmith at the market, and others as just the nameless masses of random villagers unless something brings one to the fore. I basically run a simulation of the world inside my head, and let the players loose inside that simulation to poke and prod and try to change things as best they can.

These people have goals and objectives and beliefs - again some in far more detail than others. The world has locations and people and places - again some in far more detail than others.

When players do things, the world reacts.

If you steal from an NPC and get caught then the NPC will react as suited to that NPC. They may laugh it off, they may be mortally offended and attack or demand the guards execute the thief, or they may take a note of it and extract quiet vengeance later.

None of that is me though. I know the capability, attitude and general character of the NPC. That NPC will take action as appropriate.

Yes, the PC may die (although generally I wouldn't escalate that fast without some sort of warning - but it is entirely possible). For example the NPC might have the thief publicly flogged, demand he work for free, cut off the thief's hand, or just search the thief and take any valuables he has on him.

However I make sure I warn people ahead of time that that can happen and I don't go to that as my first option. This warning happens both at the start of the campaign and if someone is about to do something very dangerous:

"I jump the ravine" "The ravine is very deep, a fall has a good chance of serious injury or death, do you still go ahead?"

"I steal the purse" "Aleyd is a skilled and experienced warrior with a reputation for a swift temper and harsh punishments and could easily best you in combat, do you still try?"

If they proceed after the warning then they can't blame you for the consequences and after a few times they will realize that the consequences are real and that adds more excitement to everything.

This appears to be a specific case of the more general issue of players potentially breaking a plot line by doing something unexpected.

In this case the issue is that if the character got caught stealing from the Captain of the Guard, a key person in the plot, the obvious consequences, being arrested and charged or becoming outlaws, might make it impossible for the characters to achieve the goal of the plot, whatever that was.

The answer to this for me is to avoid planning plot-lines (inherently relatively inflexible) with, say, a series of encounters and times, but instead plan situations with a looser collection of NPCs, timelines and places bound together with your overall understanding of what is going on.

It might seem like just a semantic difference, but actually the difference is quite profound, even though the resulting set of places, people and events might be exactly the same. You let the characters in and see what happens when they interact with the situation rather than require them to hold to a plot. Because this is loose and flexible it is less prone to being broken by the characters doing something unexpected or (OH NO!) clever. Which happens ALL the time. It also allows you to steal their good ideas and integrate them into what's happening or even change everything because what they think might be going on is much better than what you planned. And best of all, they never need to know. They think they are oh so clever for working out your dastardly story, so everyone is a winner!

The crucial difference is that a situation does not depend on the characters acting a particular way: your planning allows the players to do things way off your radar and it still has the potential to work out as intended. If you plan out a plot-line and the players go way off your radar, you often get stuck, because, if you follow the clear consequences, they lead you away from anything you have planned or thought of and if you don't it can become implausible or disatisfying.

For instance, maybe this means that they will never find the kidnapped son of the Baron (if that was the scenario that led them to speak with a Captain of the Guard) because the consequence of one of them deciding to break the law means they have to run or to fight, which might add murder to the charges. But this kind of consequence is believable but might mean that rather than follow the clues given by the establishment, they get involved in the criminal underground and come across the kidnapping a different way and steal the ransom money before it ever gets to the kidnappers. It's actually more exciting to run this kind of game as you don't know what's going to happen, which is actually always true unless you railroad the players. It makes it a good thing rather than a stress.

If you set up the situation, so that you have a good but flexible idea of what it happening, it lets the players take more of the burden of the story telling, which often flows better and is often much more fun for you as a DM and for the players.

So now let you tell me about the game I ran last week where the bugbear pooed in the alleyway to convince a guard that there really was nothing they wanted to be interested in here... first time that's ever happened in one of my games and certainly nothing I planned for! And of course it worked. Who wants to go anywhere near a bugbear poo? This from a table of 40+ years olds, who dissolved into laughter. It was something that could easily have broken a plot-line if I had set it up that way, but the situation I had planned coped just fine with a story line that was much funnier than I could have planned. And of course she earned her character a unanimously applauded inspiration point.

1. Session 0 should establish what player actions are permitted (what kind of a campaign it is) and all players should get input (though the DM gets the final call of what she's prepared to actually run), so that there's 'buy in'. This includes PvP, whether it's okay to get into wider actions that may put the group in danger with the law, and so on. This isn't to deny player agency but to make it so that everyone understands what they're signing up for; one persons fun shouldn't be at the expense of everyone else's.

   An accepted style of play in the campaign is something that should be clear from the start. The best games I have been in sort it out before session 1 and any new players get the rules when they join.

   If everyone's happy with a lawless party and the consequences it will bring with it, there's no problem to solve.

2. If "adventurers" are at all common in your world, the people in your world will already have learned to deal with the antisocial behavior they tend to bring with them (society will be wise to the classic 'murderhobo' scenario for example). Any society worth the name has preventative measures and consequences - likely harsh ones.

   These facts should be made obvious to your players as soon as they begin; it doesn't have to be exposition -- it might be almost anything - a notice board of bounties for outlaws (which the players themselves can collect on) for a variety of crimes like murder, theft and so on, or a public execution of another adventuring group for murder (with the mage and the cleric bound and gagged at the very least).

   That is, your players should know right from the very start that the society can and does respond to threats to life and property. If not stated before play begins it needs to be clearly demonstrated in the opening scene or scenes and reinforced frequently thereafter.

   You need to figure out how the society or societies manage to enforce the law when faced with powerful adventurers in a way that fits your world, whether by overwhelming physical force or magical means etc.

3. On the other hand, if you do have a degree of social breakdown with abundant lawless behavior - where the strong steal from or murder the weak - then the party should be the victim of such crimes any number of times before they have enough power to be perpetrators themselves; even if they succeed at stealing this or doing that, there's nearly always a bigger criminal who wants want they took or was connected to the person they killed.

   Such societies tend to lead to gangs that enforce their will and don't tolerate encroachment on their territory (e.g. the mafia will act to protect the very shops they collect 'protection' from, making this protection at least worth something - after all you can't collect from people who have no money left). Their punishments of people who encroach on their turf tend to be even more onerous than those of a lawful society.

Set up a few contingencies beforehand, you are thinking about it now, keep them in your bag of tricks for when you need something, even if it is several games down the line. Worse comes to worse get creative. There are plenty of ways a person could punish someone they still need.

1. I had something similar happen with the players stealing from a paladin, an grizzled not nice paladin. The paladin caught the thief. literally catching his hand. She then slammed it on the table with a gauntleted fist breaking the thief's fingers, followed by healing the hand thus healing the fingers in the broken position, all in a single round. "You're lucky kid, by the law I could keep that hand"

The players suffer a penalty to Dex checks using their hands until they could find a doctor they could pay to re-break the hand, and set it properly. And the hand never quite looked right from then on. The party cleric even refused to heal the re-break until the thief apologized, to the party. The penalty was minor in terms of play but significant in terms of roleplaying. They got a minor quest out of it, the player had some inconvenience, had to pay a little money, and got a good story. Thus Tibalt Crooked Fingers was born.

This was an extreme case but the NPC could have just as easily broke his hand and left it at that.

2. An evil or neutral NPC might have the player branded, ala captain Jack Sparrow style, the local law might even demand it. Of course the captain is not stupid, hes not going to call the guards right then, he will wait an set up an ambush for when the players return. NPC's don't have to react right then, they can plan too.

3. Or wait for the player to do the tasks, then when they are done, tell them they are not getting paid (would have been 300g). Because the thief saw fit to claim his payment (5g) early. If they complain, "( laughs) Hey you broke trust first" or worst "you're right this isn't fair, (calls guards) give me all your gold as well".

4. A less violent npc might go out of their way to ruin the characters reputation. Get them black listed from pubs or shops. imagine the players surprise when you describe that their character sees their face on a poster behind a pub counter identifying them as known thief.

5. Karma, steal from the players. Some local pickpocket might steal from the fighter or the wizard while walking through a crowd. Roll in secret for the player. they go to pay for dinner hours later and notice their purse is gone. This can even be a great hook if you want to set the players against the local thieves guild.

A few strategies not already covered in the other answers come to mind:

1. Wham! Bam! Distraction!

We had a similar thing happen during one of our very first campaigns and I think our DM handled it elegantly.

We had a rogue who thought that being a thief, he had to be thieving the whole time, effectively draining energy from the group and the story. So we had to go talk to an NPC at their house and our thief wanted (again) to go around the back and break in. Instead of punishing him or telling him he couldn't, our DM made him find the magical chest we had been looking for in the woods behind the house. Since it could disappear randomly, the thief couldn't just let it lie and come back to it later. He couldn't call out to us without arousing suspicion from the NPC. And since the chest required two hands he couldn't take it with him while thieving.

So he ended up carrying/dragging the thing to the front of the house, making an excuse of having to go pee, getting lost in the woods and stumbling upon the thing (the home owner was sceptical but bought it).

In your case you could have Aleyd draw him close at the moment of the handshake and whisper a vital thing in their ear. Or just something like "Also, watch out for thieves around town. We hang a few every week but there's no end to them." Or have an explosion nearby. Or anything as long as it fits your story.

2. Play stupid games, win stupid prizes

Another strategy would be to have the theft be successful, but instead of gold there's iron nuggets in the purse. Aleyd is Captain of the Guard after all, of course she would have a decoy purse and keep her real purse in her inner pocket.

A stolen ring could look gold but turn out to be painted wood. A backpack could be full of painting supplies.

If this happens a few times, they'll soon learn petty theft isn't worth it and hopefully stop.

3. Boomerang

Similar to the second strategy, have the theft be successful. But as it turns out the purse was charmed with a spell that turns the owner bright blue for two hours if they don't have a piece of spruce (or a similar common but not easily obtainable object) tucked inside their pocket. The PC turns blue and is an easy target for the guards who know this happens to purse snatchers, unless of course they cover up with a sheet and act like an old diseased crone.

There's an infinite ammount of silly, harmless but inconvenient things you can come up with to dish out some instant punishment (e.g. a mute spell, a rash, total loss of bowel control to name a few).

---

I think the common thread here is: don't let a thief hijack the story and wear out the rest of the party. If you can't stop the thieving, at least make it useful and/or entertaining for the rest of the players.

These are all good answers, but there's one thing you should do in addition to all of the answers provided.

Let the Player Know the Consequences Before They Roll

In real life, we know that our actions can have severe consequence against us in the future. And a real thief knows that stealing from someone of extreme importance to them for a few gold is not a risk worth taking.

But, these kinds of details can get lost in abstraction if the player is simply thinking of the mechanical aspects - they might think "I can get a few gold for doing this" and not accurately consider the consequences.

So before a player tries to make such a roll that would have dire consequences, you should say something like:

Are you sure? If you do (x), then (y) will happen, and it will have (z) consequences on you in the future.

In this case, (x) would be stealing from the Captain of the Guard, (y) would be them finding out they were robbed by a thief and/or immediately knowing that they are being robbed, and (z) being losing a ton of reputation with the town, and potentially being thrown in jail and forced to pay a hefty bail.

This is not just a good idea for crimes, but for any attempt at a skill check by a player that is likely to lead to some dire consequences if they fail (or especially if they succeed!).

Actions have consequences

the last player walks up to Aleyd, shakes her hand and tries to steal her purse. Luckily he passes the Sleight of Hand check with ease since he rolled a 16+8 (he's a rogue with Expertise).

The games basic premise is this (Basic Rules, p. 4):

1. DM describes the environment/situation
2. Player describes what they want to do.
3. The DM narrates the results of the adventurers’ actions. (Only roll the dice when the situation is in doubt).

Per your comment, you assigned a DC of 20 for the attempt. In the basic rules (p. 61) we find:

Task Difficulty DC
Hard 20
Very hard 25
Nearly impossible 30

If you set a DC of 20, and they rolled a 24 - fine! The rogue pulled it off. (See below). If you had set a DC of 25, and the rogue failed (24 being the roll+mods) an angry Captain of the Guard is still shaking the hand of the thief and notices the attempt to steal her purse! Trouble just happened and needs to be resolved. Someone just made an ally into a enemy, or, maybe some fast talking can get them out of this - "Captain, I was just showing you how good we are a this undercover stuff ..." Will she buy it?

DMing note: you can always assign disadvantage to a roll for an ability check if the circumstances around it call for that. Is there a reason that you did not assign disadvantage to this blatant attempt at theft in the presence of the mark? You don't have to, but it seems a valid use of that mechanic. From your comment, you chose not to assign disadvantage. That is certainly your call to make.

The DM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result. (p. 60, basic rulse)

If the player didn't set up the situation such that the Captain of the Guard was distracted, the sleight-of-hand check will be a lot harder than if she was distracted. There is no need to be shy about using advantage and disadvantage as a DM. It's there for you to use.

But hey, the rogue succeeded! Nice pocket picking there! This act did not occur in a vacuum.

The captain notices her missing purse

The game world reacts to the characters' decisions and actions.

Either the Captain knows who she last had a conference with, and suspects that her purse is where they are, or, the Captain (your roll) does or doesn't remember who she was last in a meeting with when she knew where her purse was.

If you set it up as the latter case, or if you don't want to just decide, make an unmodified Intelligence Ability Check, or an Intelligence (Investigation) check on behalf of the captain. (I'd do it behind the screen ... DC 10 or so is suitable here).

1. If she misses the roll (your rogue need not know the outcome) she doesn't remember them in association with the purse and thinks she misplaced it.

2. If she makes the roll, well then, trouble for the rogue.

If the party gets a visit from the captain ...

... a squad of guards (NPC, Town Guard) armed with heavy crossbows, with the Captain leading them, tracks down the party (or just the rogue) and at crossbow point suggests that the rogue return the purse.

Further consequences are then figured out based on how the party responds.

Lesson Learned for future things like this

The world reacts to what the players do. The above example is but one way to account for that.

Don't let the player derail things

Many players who are new to tabletop RPGs enjoy the feeling of liberty that stems from not being subject to a rigid mechanical engine, like in a computer game or a board game. This makes trying unconventional things like petty theft very appealing to some.

However, pulling off stunts like this detracts from an even bigger concern: a tabletop RPG is supposed to be something that you do together, as a group activity. Wanton theft and other troublemaking can quickly change the game from "a group of heroes adventuring together" to "Tuikku the Rogue stirs trouble and everyone has to fix it, every time". That's where approaches relying on in-game consequences fail --- even if you punish just the thief in question, and even if the punishment is fair, it still winds up drawing lots of time and energy from the entire group.

Therefore, as you've already opened conversation with the player in question, the next time they want to do something that'd potentially cause trouble for the entire party, say something like this:

Doing that would likely result in the rest of the session being spent to undo whatever damage or complications might happen. Given the magnitude of the decision you're making, I'm sorry but I'm not letting you do it unless everyone is on board.

(remember that everyone includes yourself. It's not something you should do lightly, but if you're uncomfortable accommodating for any player action, you have every right to veto them)

This has worked well for me beforehand --- the strong point is that it forces the player to consider the consequences of their actions on the whole game. Are cheap laughs or exercising their theft skills just for some coins really worth of half an hour of everyone's table time to resolve the incident and its aftermath? It's also flexible, as the same argument applies equally to murdering non-threatening NPCs and similar "rash" decisions.

Alternatives for sticky-fingered PCs

That said, if your player wants to maintain theft as a part of their character, you have options that don't require stealing the focus or risking the party at the table. One option is encouraging thievery as a Downtime activity, making the character earn some additional coin between adventures. You can ask the player to cover their crime sprees in a few sentences:

GM: "Ok, it's been four days since your last adventure. Do I remember correctly that Tuikku the Rogue was doing theft as a downtime activity? Roll how much money you made, and then briefly describe what you stole during that time, and from who."

Tuikku's player: "I mostly just picked purses in the marketplace from stuck-up burghers, and also swiped and fenced an ornate ring from a handsome gentleman."

If you feel the downtime action needs a little something extra, you can also drop in plot hooks:

GM: "This morning, you caught word that the gentleman has hired a large group of investigators to look for that ring --- for far more coin that the trinket is worth. They're roughing up people for information, and your fence has gone off the radar for their own safety. What's with that thing?"

You can also ask your player to come up with plot hooks or details of stolen items if you like.

this way, your player gets to engage in rampant theft and have that as a part of their character without it detracting from everyone else's sessions.

This is not uncommon since many players come from video games, wantonly stealing from and murdering NPCs. NPCs in roleplay games tend to be smarter though, and have logic.

Give them a reputational cost.

They didn't interact with anyone else. If they check their wallet after, they'll know that it was probably that thief who stole from them. So what do they do? Not fight a bunch of murderhobos. They go to the pub and complain about these so called 'adventurers' who rob and steal from everyone they meet, and then in that village or town, people refuse to serve that thief and anyone associated with them for fear of having their stuff stolen. Don't make it universal, unless they repeatedly do stuff, but if they harass NPCs, NPCs won't want to talk to them unless very seedy, and quest givers won't give them useful info.

If they had failed the check, the NPC would slap their hand away, swear at them, and march off to tell people how untrustworthy the adventurers were, unless she really really needed them.

Explain morality. Make sure they know being a thief doesn't mean they are supposed to steal from everyone they meet. Allies are supposed to be safe- just as being a fighter doesn't mean you stick your sword in random allies, being a wizard doesn't mean you turn the local cleric who heals you into a frog, being a thief doesn't mean random stealing.

Give them a common sense check. Sometimes it would make sense to steal from an ally. A sufficiently mercenary group might want to steal from an ally if there is a valuable enough possession, and that's a quest and a heist. If their ally has a rare and powerful magical item, maybe they can get it off them, and maybe it's worth the reputational cost. A few gold probably isn't worth that risk.

In a real situation, no one would ever try that sort of thing. Would you talk with someone who can probably kill you in a second and then try to steal her purse? No way.

This kind of issue can be caused by 2 things:

• The player is testing your limits to check if you are going to punish that kind of behavior.
• Maybe the player was not really aware that Aleyd can kill him easily or just throw him into jail until the end of his life.

There's multiple kinds of possible punishments. Throwing the character into jail for years is the logical solution, but the player will likely just make a new character.

Instead of this, you can:

• make Aleyd confiscate all of the character's belongings as a fine.
• start a fight between Aleyd and the player. Of course, the player's character can't win this fight, but Aleyd does not want to kill him; she just wants to impose a fine and give him a good scar on his face.
• make Aleyd neutralize the player's character, then say to the rest of the group: "You better look after him; one day you're all gonna die and it will be all his fault."