I think it's important to establish your policies as a coach for how you will interact with the team, how'when you expect them to voice opinions, how/when parents may voice opinions and just as importantly what is not appropriate and hurts the progress of the team.
With good policies in place, you can refer back to them when people don't want to follow them and explain how it hurts the team if anyone is constantly being negative. After trying to change the bad behavior I think there is a point where you are totally justified in letting the parent know that their disruptive behavior may cause you to either ban them from attending practices/competitions or even let them know it may ultimately cost their child the spot on the team.