It never ceases to amaze me how professional some of us claim to be, yet can resort to the most unprofessional conduct when a customer fails to pay.
I had a customer who I dropped recently. She was always desperate for clean windows, yet when it came to paying she always had different excuses for avoiding paying. After a while, enough was enough, so I simply walked away and cut my losses.
She recently phoned a friend of mine who is also a windy, wanting her windows cleaned. He mentioned to me he had had a call from this lady, so I told him about my experience with her. When she phoned him again later he told her in no uncertain terms that her reputation among window cleaners was bad, and that she would be well advised to pay me what I am owed.
Of course she still hasn't paid, and I know she won't, but the point of this is that her chances of getting her windows cleaned by anyone now is virtually nil, while I am still picking up better quality customers and building a good reputation.

John