I operate quite a few forums and honestly, there's nothing that can be done about it unless you want to waste precious time hunting down post-builders.
I think a lot of this junk has to do with forum posting services such as the one advertised in a paid review on john Chow's site
Going Organic with In Forum Advertising | John Chow dot Com
Considering most forums (such as mine) do not allow users with under 10 posts to post a link in order to combat spammers, they are using new tactics like copy/pasting from other forums while they put the links they're promoting in their signature.
In my opinion, it's just an occupational hazard of operating a forum.