Plugin/Hack for adding text to comment?

I’m looking for a way to deal with troll comments other than outright blocking/deleting the comments. I want the comment loop code to insert a banner statement at the top of comments left using some unique identifier (IP address &/or email address). For example, if troll@jerk.com keeps derailing discussion then I could flag that user so that whenever he comments a statement like this one would appear: “This user is a troll. Please beware that engaging the troll will only waste your time.”

Thanks!