The following mod only requires changes to the register.php file so it will be easy to maintain through 1.2 updates I'd expect
Since installing the above mod on one of my boards there's been no spam registrations in over a week (previously getting 5+ a day).
I'd also suggest asking new members to verify their registration through email (found under Admin > Options) though I never found this to prevent spammers.
To stop spammers you're going to have to make some code changes unfortunately, the above mod is the easiest to maintain imo. You can see a demo of the mod on my registration page here. Spam question edited to suit the site