I started a forum a couple weeks ago on Simple Machines and have the Captcha (or whatever) enabled and I'm getting spammers like crazy. I ban the IP's, ban the e-mails (banned the whole *@mail.ru) and they're still coming in.
I set up an extra verification question pertaining to motorsports that ANY human being in the civilized world would know the answer to if they'd be interested in racing at all. Something like: ____ Earnhardt drove the #3 Chevrolet. Of course the answer is "Dale", and even people who don't pay attention to Nascar probably know that from the question... but bots don't. And since then, there's been exactly ZERO spambots registering.