My default expression in situations like these has become a look of intense confusion. After years of waitressing I've become very non confrontational, but even at work I can get away with a look of confusion. They may interpret my confusion however they want, but in my mind, I have the satisfaction of knowing that it means "how have you managed to live so long in this world without learning any manners?" or "is there something wrong with you that makes you think it's ok to act like a child?"
Here I would be thinking, I did not tolerate that from my "child" so what makes you think I would tolerate it from an "adult"?