The spammers will find a new way around, eventually, but it should hold them off for awhile!
The only board where I don't have any spam trouble is a mostly private one, where new members have to be voted in, and we keep registrations turned off completely. We just turn them on when we know we have a new person coming in, and then turn them right back off again.
Obviously that's not a good option here, but there really isn't anything that's 100% for keeping out spammers. We'll just have to stay on top of it as much as possible