This is for all of you who run web forums yourselves...
And I promise I'm not just trying to plug this particular forum.
Anway, over at
Pirated Sites (a cool little forum for exposing pirated web designs) they're currently being assaulted by spam - if you open the "General Discussion" section, every single thread there that doesn't have a title along the lines of 'companyx vs companyY' is almost guaranteed to be an innocent looking thread with a link to www . deluxe-planet . net in it.
Essentially, I'm pretty sure the admin there wouldn't be too happy about disabling guest posting (most of the legit posts are made by guests anyway), so how else do you deal with this kind of thing? It's getting really annoying..