Do you know what is more fun than firing a bad employee? Or a bad supplier? A BAD CUSTOMER!!!! Write a speech and practice, practice practice.
"Dear POS customer, The problems you are claiming we have caused you are very distressing to us. It is odd that we have done such a poor job of taking care of your cleaning needs, and yet this is the first you have told us. As our quality of work does not meet your needs, we are ending our relationship. Perhaps someone else can meet your needs, we cannot. Good by." Turn and walk away.
The first time I told a customer to make other arrangements, I was quaking in my boots. Now, I just say, "Call someone else" The look on the customers face is priceless, and a real ego booster to you.
Often, they will deny any problems, and beg you to stay. That is when you hit them with the ******* tax, 20% increase. That may make them bearable to deal with.
Try it, and let us know how it works.