I run another social news site (realestatevoices.com) and captcha doesn't help. The people inputing these links are 'SEO' firms based out of India.
Validating email accounts is worthwhile though. But keep in mind these spammers are actual people not automated in many cases.
interesting site...how long have you been up and running?
Not all spam is automated.
You are right; at the same time there is nothing novel about CAPTCHAs. I also have a habit of running wild with ideas, and I suppose it's beginning to show...
But hopefully my hatred for spam is showing through as well!