I believe in the top right corner of the thread, you click 'unsubscribe' ? I'm not certain but I believe that's how you do it.

Edit: click on 'Thread Tools' towards the top then hit 'unsubscribe'.
Haha I really need to get around to doing that as well. I always get freaking emails for the 'This or That' or 'Something You Don't Know About Me' thread. I just can't be arsed to fix it haha too much effort required there :P