If you have been in business for any length of time, you probably have come across that difficult client. Sometimes a difficult client can become your greatest asset. They may be acting that way because they have been treated badly by a competitor. In that case, all you need to do is supply excellent customer service and you could have an advocate for life.
Some difficult clients, however, need to be fired. Yes, fired. Do you really want to keep dealing with that headache? How much time does that difficult client take up? Are they causing you undue stress that is affecting your business? Not everyone works well together. Free them up to find a company that works better with their personality. You could even suggest one or two (not as a punishment to your competition, but as a real courtesy).