The 80/20 rule states that 20% of your customers account for 80% of your hassles. Try as you might, you just won't be able to please all of your customers.
Before you 'fire a customer' remember to talk with the customer and find what's bugging them. With the customer, write down what you can do to rectify the problem. Work through the customer requests. When finished ask the customer if they are satisfied.
A few customers will take your accomodations as a sign of weakness. They will continue to push you for even more. These are customers you don't need.
You can read more about this 80/20 rule in The Guerrilla Marketing Handbook by Jay Levinson & Seth Godin.