Spammers have been targetting phpBB/SMF quite a bit recently. One thing you can do (if you haven't installed this mod) is to install reCAPTCHA (
http://custom.simplemachines.org/mod...x.php?mod=1044) - that's regarded as the best CAPTCHA out there.
Also, anti-bot puzzles are invaluable - these are standard in SMF 2.0, and although it's only in RC I'd fully recommend the 2.0 branch (am a beta tester of SMF software). If not, look for an anti-bot puzzle/question mod for the 1.1 branch.
Bots have got very clever recently, and can break reCAPTCHA sometimes, although nothing can really break through anti-bot puzzles/questions (I ask things like: "What is 2 + 5 - 1, answer in words?" and things like that etc)