i have to deal with that kind of crap all the time in sales. if a person is so set in thier opinion there is little use in trying to bring them around (there is an analogy about argueing with a crazy person that woudl be very apt here). much od the time that are simply looking for someone - ANYONE - to attack and vent thier bile upon and you just happen to be in thier cross hairs.
A very simple "Thank you for sharing & have a good day" in my most sincere used car salesman voice usually works for me. yes its patronizing as hell but it send them a very clear message that you have better things to do than be bothered with them.