We have a somewhat different approach to preventing spam - something that's been very successful and which even Sitepoint was very keen on examining in some detail with the view to implementing on their site. There's a thread or two about it, like this.
In short, we allow anyone and everyone to register, even if they are known spammers. I figured there's no point in deleting them manually. Instead they have to prove themselves for the account to live. If their initial posts are spam posts, the post is deleted but nothing happens to the account except that it doesn't get to prove itself and gets therefore destined to be killed in the monthly cull.
So, not all registrants know how it works and some sign up to see if they can slip a spam link. OK, their motivation is to spam. But hundreds of others just sign up and don't do anything. They don't attempt to make any spam posts, they don't attempt to drop links, nothing.
And I'm mystified by that. Any suggestions from you forum experts? In the last couple of days or so we've had all of these:
I'd be surprised if I got one poster out of all of them. The accounts will get deleted in due course. But why take the trouble to sign up, jump through all the manual stuff like Captchas etc if you have no intent to even spam!?