Spammers are now outsourcing for manual spamming (via Bloggers Blog) to workaround CAPTCHAs. Though a single spammer cannot be as fast as a machine, thousands of them together than spell a lot of trouble. Captchas force you to identify certain characters and enter them, which machines cannot follow. This works fine assuming that humans, for whom captchas are built, do not spam. The outsourcing trend will overcome this. I think the problem here is with the basic SEO behavior. The websites for which spamming is being done should be punished and excluded through all indexes. This will harm the basic purpose for which they employ spamming, and can help the situation.