I don't have many bad'uns, but I always include in my letter, that I will "register the debt on "The National Window Cleaners debt register", and she may then find it difficult to obtain the services of another window cleaner in future"
Yes, I know there isn't, but they don't, and I find it brings a result with about half of them.
If it don't, then walk away, [and warn the other 'cleaners in the area]
Baldeagle