pet peeve - unable to talk back

Customer is always right. I don't think so but that is besides the question. I work in a customer service industry so I have to adhere to that. In some instances, there are just plain mean and unreasonable customers that I encounter. And what bothers me most is I can't talk back to them even if they are verbally combatant and abusive. I can't tell them what I really think of them or the situation. I just have to shut my mouth up and take it. If I don't, I might run the risk of jeopardizing my job. And I hate that feeling of helplessness.