Sure it exists. We've banned people for it. I've scolded them publicly for it, too, in the hope of discouraging others from doing similar things. So no, what I'm saying is that it's wrong to claim astroturfing without evidence, and the mere fact of someone posting a comment that you feel is wrong—even outrageously, perversely wrong—is not evidence of astroturfing. It's evidence merely that they have a different view than you.

I can't speak about other forums but as far as I'm aware there's one outfit that sells HN votes and comments and stories. What their customers don't seem to realize is that using that service will get them banned here. At this point I figure the spammers know this too, but since people still buy from them, they probably don't care. I doubt they get much repeat business though.