Those in customer service, hospitality, or related jobs; How do you deal with Jerk customers?

When a customer or guest is a real PITA (pain in the ass), or a complete asshole. What do you usually do?

Today at the hotel I work at these rather elderly folks had me move them to the ground floor because they could not walk up stairs and we have no elevators. We are very full tonight, and I had to bend over backwards to move some rooms around and get them into a ground floor room. I even gave them a coupon for a complimentary glass of wine to make up for the fact that it took me some time to get it figured out.

Then the man misplaced his suitcase. He thought it was in the lobby. I told him I had not seen it. He kept insisting that “it was right here! it was right here!” I suggested other places it might be, and tried to be as helpful as possible. He said “I’m going to call the police, You people are terrible!” after I had bent over backwards to be as helpful as possible. Shortly afterwards, he came back to the counter to tell me he had found it.

Have you had similarly awful people in your workplace? What did you do? What would you have done? Anecdotes encouraged!