Sooner or later in most businesses, you have to fire a customer.
The relationship may simply be a bad fit. Those are the easiest situations. There can be amicable and respectful separations. But the usual break is less direct and far less pleasant. Some customers are impossible to please, some are flat-out rude, and a few are crazier than bedbugs.
The biggest mistake is to refuse to sever a relationship because of money. As I tell my classes on harassment prevention, would you want to tell your employees, "You know, if someone pays us a lot of money, we let them harass you?" A public works executive put it even better: "We wouldn't let someone abuse our machinery. Why would we let anyone abuse our employees?"
The decision to fire the customer is often a matter of self-respect. Some customers skirt the edges of improper conduct, coming close to the breaking point, and then backing away. A few are so skilled at doing so that it is natural to suspect that they enjoy it. You are fortunate if you can spot that type before you commit to the project, but once you do, if at all possible, drop them like a hot rock.
Life is too short to be a door mat.