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

If you are running this world on a multiplayer server, you can disable command blocks completely via the server properties. Shut down the server, open server.properties with a text editor of your choice and set

enable-command-block = false

This disables all command blocks. Join the game, destroy the command block in question and reenable them afterwards.

In singleplayer, it's a bit more complicated. You have to open the world as a multiplayer* server. Open server.properties and set

level-name = [relative path to your savegame]

e.g. saves/world (use / to divide folders).

*If you haven't run a multiplayer server yet, you can download the server.jar (or server.exe) from the Minecraft website and save it wherever you want. Run it once to generate some files, including server.properties and a savegame called world.

To join your own server, connect to localhost or 127.0.0.1 in Minecraft.

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.