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. Read more here. You can read more about this 80/20 rule in The Guerrilla Marketing Handbook by Jay Levinson & Seth Godin.