I have this problem with the dragon eggs thread.
On top of that, I think I have heard moderators say that they prefer that people not delete their posts, so I never have tried to go back and do that.
But it does make me wish there was either a way to have those threads stop showing up in the "new replies" section, or that the really long off-topic threads could be periodically locked and restarted.
Even so, though it's mildly annoying to have that thread continue to show up, it's not honestly that big a deal to just ignore it and periodically mark it as read.