67% joking because everyone knows the message board era is over and I will never be able to delete a popular thread.
23% not joking because I would delete personal information people didn't want revealed, underaged boob pics, or similar things that might be popular.
8% joking because I'd have to sign in to the admin area and that's enough of a hurdle that I probably wouldn't do it just to be a dick.
2% not joking because I am sometimes a dick just to be a dick.