Regardless of where you work, it's simply not acceptable to be rude to paying customers. I did my time at McDonalds and I suffered through extremely rude customers. Little did I know, the worst was yet to come! I'm now a barista for Starbucks, and I'll tell anyone that the evil customers of McDonalds are NOTHING compared to the *** holes who drive in, BMW, after Mercedes, after Lexus, after Cadillac... You sincerely greet them and they don't even aknowledge you, and after they MAKE UP a drink that doesn't EXIST, they get pissy with you because the OTHER Starbucks makes it right, and I didn't.
Point is, it doesn't matter how rude someone is, as someone in the ****-ant customer service position, it's your job to be nice and accomodating towards customers. Don't like it? Go to college. (Not that it really matters, I work with people that have degrees and they're still stuck as baristas, but everyone has to have at least one customer service job in life...)