Depends on the type of spam. If they're automated registrations you'll need a captcha of some kind to block them, which CB AntiSpam can provide. If they're manual registrations from a human spammer there isn't much you can do to stop them as they're just a person registering and only way to really deal with them is see if their registrations have a pattern (e.g. same email domain, ip address range, etc..) and block the pattern.