Shockingly, image verification is of little help. The good bots can breeze right past it using the same AI technology that is used for handwriting recognition on tablet/palm computers, and face detection in digital cameras.
The thing that has all but stopped the spam bots here is the new "trivia question" that must be answered on registration. Start to register a new user here and you'll see it. This is customizeable in VBulletin, and you can code one in phpBB if you're savvy. extremecarving.com uses phpBB and they wrote a trivia question into their registration page.
Most bots see this and don't even try. Some still do, and they get the question wrong. However in VB that by itself wouldn't prevent them from registering and posting, so you do have to turn on moderation for all new users. Then you will see who got the question wrong when you go to approve new users (a daily chore).
Bottom line, nothing can stop a human spammer, but I'd estimate they are less than 0.1% of all the spammer registrations we've gotten.