Eh, all of the forum spam I get is from "users" who have actually created accounts and sometimes posted halfway legit comments (it's a music theory forum, and they obviously don't know anything about theory, but post something that tries to fake it) before starting to post spam.

The best solution is removing the incentive (though I've had rel="nofollow" auto-added to all links since it was first proposed, to no avail...).

and then you get false positives. I was somehow trapped by their spam detection, even though my comments were totally legit, I've had a good 'track record' etc.

It doesn't tell you anything about it, the post author wasn't notified either.