Go round to see him face to face and very calmly ask him why he feels he doesn't have to pay
Whatever excuse he comes out with, say ( very politely ) I'm very sorry about that, I hope you have better luck with you're next cleaner. I'll just put back the dirt I cleaned off and I won't bother you again
Then get pole out of van dunk the brush in a bucket of water then some mud and start smearing it on windows
You'll be surprised how far a bit of mud on a wet brush will go
He will either stop you and pay you ( result ) or let you do it ( result , cus you'll feel better ) or try beating you up but you already said you weren't worried about that, so it's win win