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Gizmodo has one of the most interesting comment policing system, and for the most part, it works well.
The idea is to have a 'core' of people who promote or demote comments. The idea, as I understand it, is to have it so as you gain more prestige on the system, you're allowed to also help out with policing by killing off trolls, useless banter, etc. It's sort of a crowdsourcing/democratization of policing comment which creates a website 'norm' through its community.
I think part of it might be technological limitations as well as non-technological limiations to an extent, but I feel that there is always a solution beyond pure banning.