I think this situation is alike to that of a bully in school. How do you stop someone like that? You can (a) confront the person, but I think that would, most likely, escalate into a flame war and all the horizon of civility and politeness would be crossed much faster than any of us would like to; or (b) you can ignore the person. However, that strategy also has its problems. It might stop the bully, but it also might give him confidence to pursue his abusive behavior without consequence.
Definitely just not responding to negative comments is the best--but also the hardest--thing to do. I know that a PM service is in the works. Maybe by default ignored users shouldn't be able to send you a PM. Just spitballin'. At least that way it would prevent spam to some degree.