Begin by telling the interviewer that thankfully the majority of customers are pleasant to work with, and the rude customer situations are typically few and far between. Next, say to the interviewer that you understand customer service means always having a smile on your face, cheerful attitude, and pleasant demeanor but you too have your limitations. Show that you can be assertive when needed while remaining professional.
"If a customer screamed at me, I would hope that management or security would step in and ask them to leave. I am all for customer resolution; however, I do not accept abusive behavior. If possible, I would respectfully walk away from the situation. It's important to remain composed and professional at all times."