Yes, the ones with unrealistic expectations. Send them down the road.
I don't want to suggest you shouldn't provide excellent customer service to difficult customers. I do, and win them over. They're money is as good as the next customers. I don't know how to define how I tell the difference between a customer that I have to win over that will become loyal, with one that's a PITA and I'll blow off (and I do blow them off). But there is a difference. They'll both make demands, the second one will call on the weekend though. The first one is busy with their family on the weekends, too busy to call me.