I'm a utility worker / grocery clerk
I can relate in someways on how you feel. I had people get upset at me over stuff my store doesn't have, and other bullshit. It can get annoying as hell at times when I'm trying to focus on my job. It can really get annoying having the same person asking me questions over and over again. Especially when I'm trying to focus on getting stuff done on time.
One time I had to control myself when a customer blocked my cart and called me boy, and I couldn't get my stuff done because he was in the way. Even then I had to remain calm without doing anything to piss-off the costumer. Hell I almost could have gotten fired afew days ago, because I made a silly mistake. Last thing I need is to have costumers complaining about me. Anyway I'll always respect people and be nice since its my job, but if people start getting violent then fuck it I'll hit them back.
Once we had a particularly nasty customer who INSISTED that we had a five year old blackberry phone in stock and that we were just hiding it from her, and then she spent ten minutes explaining to us that if we were doing our jobs right and weren't so "fucking useless" we would have the phone in stock at all times. I said "You know what, let me double check in the back." I went in there to get a minutes peace and calm down before I had to deal with her again, but I just thought fuck it and sat down and watched ten minutes of a Quick Look on my phone. I then went back out to her and told her I checked the whole store room and we didn't have one. She actually seemed happy then, and said "Well at least you bothered to look and didn't keep wasting my time."
After she left we laughed for quite some time.