I wasn't so much worried about the spammers finding their way back to me, but my spam finding its way to you. :) (via infection of course)

The authentication link is a good idea. They can still post non-automated messages (and automated messages if they find the right lib for breaking captchas) but at the very least it will give you a way to keep a close eye on suspicious activity. Someone starts triggering the captcha? Take a look at what they do.