Love this blurb from this blog:
When you fire a customer and politely ask them to move on, you are withdrawing yourself from their trollish dance. When, instead, you focus on the good student, the worthwhile investor, the delighted vendor, you improve things for both of you. The sooner you get back to work (your work), the sooner you can move toward your best outcome, which is achieving what you set out to achieve in the first place.
The real tragedy of the person who dumps on you is that you pay twice. The second time is when you get bent out of shape trying to get even.