"Tell me about a time that you disagreed with your manager. What did you do about it? What was the outcome?" I hate this question. It's a trap. Of course, at some point in time, you will face this dilemma, and your manager ultimately just wants you to shut up and carry on.
I always answer this honestly, "Yes, I did disagree with. I took the complaint up the chain, and pleaded my case, only to feel like I won a battle, when really I lost the war." Lesson learned, the grievance policies put in place are just a way to make you feel better. Most likely, you pretty much don't have a case.
According to the shift manager who interviewed me you go and get your shift manager so they can handle it as putting your self at risk is not part of your job and in fact they consider dealing with majorly upset or problem customers to be the job of management. Basically say you will elevate the situation to the your superior.