One other course of action, which invariably puts these people on the run is to quote legal statute ate them. Something along the lines of "Oh, you may think that your comment is acceptable but you are in breach of the Protection from Harassment Act 1997. If you continue in that vein I may have to bring this to the attention of the police" Since most people don't know much about the law the fact that you can quote such things is usually enough to dissuade them.
I havent needed to use the harrassment laws but have a good working knowledege of Sale of Goods and related acts and this has served me well.

FYI summary at http://www.solarnavigator.net/embass...t_act_1997.htm

Slainte!