Bike renters overextending seatposts

I assume they are extending the seat post that far because they need it for an appropriate saddle height.

So the solution is simple: Offer bikes with larger frame sizes or at the very least equip the bikes with long seatposts which can be extended far enough.

Any other solution would force your customers to ride with a too low saddle.

If you still want to make sure that they can’t be extended too far, just in case a giant with long legs comes along, there is a solution I can think of: If you have seatposts where you can see through the tube (common on cheap steel seatposts) you could try to attach a string (or steel cable) somewhere at the bottom bracket, thread it through the seat tube and seatpost and tie it to the saddle rails. Could lead to some noise though if the cable rattles inside the seat tube.

If these are beach cruisers or similar, they'll typically have 22.2 or 25.4mm seatposts (7/8" or 1"). If so, putting longer seatposts in all of them would be cheap.

If you haven't already, make sure you're hooked up with one of the wholesalers that cater to your type of business. In the US, the behemoth is JBI, but there are others.

Note this only applies if the kind of bikes you're renting are some kind of casual, one- or two-size thing. If we're talking about more serious bikes than yes, the answer is stick to appropriate sized bikes.

I think the best solution would be to use a seat collar with a security bolt and then pre-adjust the seat height when they check the bike out. Perhaps have some seats on the ready with extra long seat posts for people who need a higher saddle height.