I'm stuck in a teleporting loop. What can I do?

For situations like this, an external world-editing tool is best. Find the command block, and delete it.

A common one is MCEdit, which is available here: http://www.mcedit.net/

As @MrLemon mentions, you could be running a multiplayer server, in which case MCEdit is a bad idea -- the download time of a 60GB map is non-negligible. In that case, follow his answer.

If the command block is not at the spawn; type /kill.

But if one is at spawn just look down real quick and break it if you can.

Destroy the block using a world editor, then use a command block to comparator for the test part with the /testfor command.

You want the comparator to go to a command block that just puts up a message to find out if the /testfor command and comparator are working properly. Do not start the /tp command until you have confirmed that the first two parts are working in conjunction with each other properly.

You could change the gamerule randomTickSpeed to 0.

Something similar happened to me: I was being teleported 5 blocks up with a command block in a Minecart on a powered activator rail. They activate every 4 game ticks.

If all else fails, use /summon PrimedTnt to blow up the command block.

Another alternative is to change the command using NBTExplorer. This tool is much less powerful than MCEdit, but it easier to use and does work for this case. This works largely the same as my answer here, so feel free to reference that in case something is unclear. The crucial thing is that you have to remember either where your command block is located, or any semi-unique part of your command.

Command Blocks are Tile/Block Entities, since a regular data value is not enough to store all information about them. Instead, they have NBT data similar to entities, which are detailed on the wiki. In particular, command blocks ("Control") have a Command entry. NBTExplorer is able to read the region format and search for a Tile Entity with a specific entry.

1. Close Minecraft. Open up the program and select File > Open Minecraft Save Folder. You will get a list of folders corresponding to your worlds. Open up the world you want to search, and select the "Region" folder.

2. Use Search > Find (Ctrl+F) to look for Name: Command and Value: [your command snippet] until you have found the command block in question (This is described in greater detail in the answer linked above).

3. Double-Click the Command entry and change it's value to either fix the command directly (for example, turn kill @e[type=!player] into kill @e[type=!Player]) or just enter something harmless (e.g. leave it empty).

4. Press the save button and close NBTExplorer. When you open up your world in Minecraft, the command should be changed.

/fill X1 Y1 Z1 X2 Y2 Z2 minecraft:air

Fill in X Y and Z with the approximate coordinates of the command block, and you've replaced a region around the command block with air.

Step 1 figure out the coordinates of the command block that is teleporting you

Step 2 do the following: /fill (x coordinate of the command block) (y coordinate of the command block) (z coordinate of the command block) (x coordinate of the command block) (y coordinate of the command block) (z coordinate of the command block) minecraft:air

For example, if the command block is at 34 55 -2769 do /fill 34 55 -2768 34 55 -2768 minecraft:air

You could turn to Creative mode and fly so it can't reach you if it is below you as a pressure plate... I don't really know if it'll work, but you can try it! ^_^

For Bedrock Edition, just type the following command in chat:

/gamerule commandBlocksEnabled false

This will disable command blocks. Then type this when you are in a safe position:

/gamerule commandBlocksEnabled true 

This will re-enable command blocks.

What you could do, is go into the chat, and use the set block command to set the command block to a block of air.