Here's one I used once at an old job:
A lady came in to the convienance store I was a manager of complaining about the price of our gas (which by the way was the same or less than anywhere in town). Keep in mind this was at the end of a 14-hour day. I told her, "Don't blame me, blame the environmentalists." She replied, "Well, it's your store! YOU should make it cheaper!"
My reply: "Sorry, can't do it. By the way, you owe me a rib."
When I was working as a waitress at ChiChi's back in the day, there was a girl I couldn't stand. She came in one day with a fresh suntan/burn and I said "You know, you can't tan away ugly".