If it's REALLY SPAM or some offtopic stuff, I'm flagging it as no index.
If it's something helpful but outdated, I'm deleting it, but not without creating a redirect to the new helpful content!
It's IMO going to harm you a lot if you delete a topic that ranks very high for your relevant keywords, especially if you're also getting many visitors for it, either from the search engines, or other sites that link to it, so I'm always trying to create an useful redirect target for it.