If someone could do this it would make my forum awsome! There was a mod like this for 3.0 that basically stopped spam dead by taking away their reason to spam - posting URLs. No matter what many people say, Captchas don't really do a damn thing to stop spam because most have already been broken by spammers. They might take the edge off, sure, but the real thing that stops them cold is preventing them from posting any post with a link until they have a few posts. New users just don't tend to just show up and start posting URLs, so this method, in combo with a good blacklist, stops almost 100% of spammers. The only ones who tend to be left are the human spammers who come in, act awkward and then just give up.
Seriously, on my old 3.0 forum there is no spam. There used to be twenty or more spam topics a day, now I get one about once a year, if that.