You may want to put a custom profile field that is required at registration, in a pop up menu, I put that in my forum as

"Are you a filthy spammer?"

and a pop- down menu comes down with No selected by default and the user must select yes to register, and if they select no, they are told to choose the correct answer.

It solved most of our spam problems, we have no spammers, on a forum with 200,000 posts and 2000 members.