Why not grab the users Mac address and IP and award them a badge based on each comment they leave? A disqus like startup can add such a system and the badges would range from extremely positive to extremely negative (hateful useless troll).
Perhaps an intermediate site, that you log into with an OpenID, and which then gives you a new OpenID that has some form of ranking/karma system enabled?
Or- Just a standardized method to track karma/ranking across disparate sites? Maybe an extension to OpenID, feedback for feedback?
The only obstacle I see, is how the information would be conveyed. Without the participation of the hosting site, it would be little more than a static link.
Maybe a tangent of Flattr's offerings?
I guess even the IP address since it's based by router would not be completely effective, especially in large companies. Though it could gather that there a lot of trolls at one company or organization. Thinking of recent Yahoo/Skype debacle and that guy losing his job over trolls.