There's a hack called "NoSpam!" that does this.

The only problem with modifications that ask questions, is there may be users who are legitimately unable to answer them. Whether it due to diminuished capacity or language barriers. CAPTCHA's are international and involves little more than simply typing the letters you see on the screen.

However, if you have no desire for anyone with any of the previous detriments on your forum, you have no reason to concern yourself with those issues.

It was for these reasons that I incorporated reCAPTCHA into vBulletin, to replace the default CAPTCHA. It's whole existence is based upon the fact that OCR systems cannot read the text. But, again, this would only be useful if your trying to attract a diverse userbase rather than a specific one.

Whichever route you go, just be careful not to go overboard. I've (attempted to) register on a few sites where there were like 30 spam-checks in place.. once I saw that, I just opted to not even register to begin with. *shrug*

Try and find a common link between lots of spam accounts, and Prune users based on that?