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.
/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.