[dchuk]

Having done some link spam (in decent volume) in the past, your best bet in stopping it would be using more sophisticated captchas (such as the ones that have you identify a cartoon in an image or something non textual) and identifying the system posting the comment/spam link and seeing if they're a real user with a real browser or just an automated spambot like xrumer or scrapebox.

You can also turn off links in comment bodies and the URL field of the comment form to try and prevent scrapers from even finding you a worthy target. Won't help identify spam though.

Finally, centralized spam identification systems like Akismet work really damn well because they are watching the whole site network at once and can use those heuristics to identify spammers rather than the actual spam content itself.