This might be worth a look: http://www.spambotsecurity.com/
"Through a new protection subsystem, ZB Block has over 40 new signatures to detect the BEHAVIOR of the latest "HTTP_POST" attack bot families. These behavior detections, along with the weaker but still important detections of the 9 most popular "HTTP_USER_AGENT" names, allow ZB Block to essentially kill off these bots. ZB Block is compatible with the most popular CMSes, Bulletin Boards, Forums, and PHP Scripts out there, and protects from thousands of older bots and exploits not only by name on some, but they way they attack YOUR site."