The problem I am finding is a little piece of software called xrumer 
It is a spam bot which can post to vBulletin.. It has hit AGD and GPWA for sure. Not sure about CAP.
They typically post small replies to existing posts and use a signature link. Replies like ‘bump’ or ‘my 2 centsbump’ and crap like that are common, but of course that probably depends on how the idiot set up the software.
I have a hack which forces users to post at least 15 times prior to signatures being active and I have removed the ability to see signatures elsewhere to keep the linkbacks from happening.
It’s a real pain in the butt lately.
The software can hit a variety of forums, including (from their details page): phpBB and PHP-Nuke with any modifications, yaBB, VBulletin, Invision Power Board, IconBoard, UltimateBB, exBB, phorum.org, wiki. It breaks captchas and auto-registers with auto-activation as well.
The last feature I know is that it Pisses me off!