Find a phrase that you can you can use in the course of a conversation. In your mind make that phrase mean something totally different. Such as "I understand." comes out of your mouth but in your head you hear "What a freaking idiot!"
Got me thru working the return counter after Christmas and working at a bank, as well as working a 1-800-number for collections. (People get really upset when their money is involved!)
Just take it with a smile. No matter what you do, you're going to have those customers. They can't complain further if you don't give them a reason to, and are really friendly, despite their rude behaviour..
Just smile, remember that everyone has different personalities & not everyone is friendly & easy going. No ones asking you to be friends with all of them, just serve them, tolerate them & move on :) Patience & tolerance.
Have you ever watched the show Hard Core Pawn? They have their rude customers kicked out of their store, because they are in the way of better customers that they will make money on. Don't know if this will work in your situation.
Just remember that you're paid to be there. They are wasting their own time by being rude and difficult. Do what is in your job description and no more. I would always just say in my mind "at least I'm earning money by putting up with you" do what I had to do to passive them and walk away. If they're particularly difficult, get your manager to deal with them. Don't let jerks jeopardize your job by baiting you into reactions that are inappropriate.