To elaborate on my analogy; whats keeping people paying to get on the train? Because they're all nice people? Hell no. Its the perceived threat of punishment for riding the train without a ticket, however unlikely that punishment will be, hence people simply follow the rule.

If a minor punishment were to be applied to excessively rude people (specifically those who either grief or personally attack others), such as a decrease in rewards, there is the possibility people would act as most people do on the train. They simply avoid being punished by not being a dick. Those that chose to be mean play their chances at possibly being punished by a random "judge/patrol" (such as in my original suggestion).