Being deluged with porn/pharma/gambling comment spam on my homespun forum site, I recently deployed a captcha check. This stopped 95%+ of it, including the new and very annoying hundreds-of-links variety.. but a few posts are still getting through.
So I'm wondering.. is my captcha being decoded or are there people out there paid to sit at their keyboards doing nothing but posting obvious comment spam to forum sites? I can see it now.. a comment spammer paying accomplices a penny a post.
Image verification can be defeated, and spammers will indeed sometimes hand authenticate. You could try a different image system - if it kills it, then it was automated. If it persists no matter what, it's human.