Maybe I am a soft touch, or am just too laid back, but I have written off hundreds in the past, and will probably continue to get caught occasionally, although most of the dead wood has now gone.
I reach a point where I realise that I am not going to get my money without a load of stress and hassle, two things which I just don't want in my life, so I simply walk away and write it off.
I never think about exacting 'revenge'. Seems to me that two wrongs don't make a right, and furthermore I would only damage my reputation if I was seen to be deliberately defacing other peoples' property, particularly if such actions got back to my good customers. Definite career own goal, if not suicide, in my humble opinion, and not an ethical way of getting payment.
I do, however, wonder what these ex-customers will be thinking as they see me time and again cleaning their neighbours' windows, and bypassing theirs as they get ever dirtier.
John.