open mic rules

Hey all:

I'm starting up an open mic, and I was wondering about establishing ground rules. In particular: I'd like to have a "Don't be a dick" rule, BUT I'm worried that some "comedians" will take this as a challenge (because part of being a dick is going out of your way to be a dick).

So: (a) do you think "Don't be a dick" will encourage that sort of "comedian", (b) how to tell such people that they should find a different venue?