Would it be helpful then to have a special type of comment that can't be voted up (zero points always)? So, for the troll rebuke, have the rebuke marked so that people couldn't vote it up.

This way it would appear in the users's comment's page, but would be down-modded to the bottom of the page, with the trollish comment?

Maybe just include a "no points" button or checkbox on the comment edit screen?