We tell our DD to tell the child who is doing the unacceptable behaviour to stop (with her hand in a stop motion facing the child) because that hurts me or I don't like it (something along those lines). If the behaviour continues she is to come to me. I have no issues telling a child to stop a behaviour that is anti social, will hurt a child or themselves, put a child in danger/themselves. I take Kazbah's approach. If the child wouldn't stop, I'd then seek out the parents.
I wouldn't tell my DD to push back b/c that is the behaviour you are trying to avoid.