Ignore them. Return their boorishness with a smile and a calm demeanor. Their problem isnt your problem.
How to deal with mean and rude people?
Ignore their behaviour and understand that you don't HAVE to acknowledge their rudness .. However, if you choose to .. Simply say your piece without making a big deal out of it and let it go.
Most times you can't. I make it a habit in life and on here to completely avoid them. In public I quickly walk away. Sometimes I state my point and move on.
It used to upset me when I was young and then one day I realised that if a dog barks at me I don't get upset. It's just being a dog.
The parallel? When mean-spirited people bark at me, they're just being true to their own nature. Once I stopped taking it personally it stopped bothering me.
Mind you, I don't lie down and let them wipe their feet on me, either. >:-)