Dont clean them again
Stick a letter through his door asking him to pay
or my favorite....
stick a letter through his door telling him that if he doesnt pay then you will pass his debt on to your nominated debt collection agency or claim the debt back through the small claims court.
Ive done this twice now... both 2 cleans - 1 refused to pay out right, the other kept telling me she paid (her laptop magically died when i went round to see her record of payment). Stuck the letter in the door and as if by magic the money appeared in my account on day 7
Obviously i havent cleaned either of their windows since... but if your not bothered about loosing them then its a nice way to do it for people who have a nice house and nice cars and the neighbours think they are loaded.
If you really want to p them off then put the letter addressed to the person in question and the envelope (not sealed) with just 'owner' on it and stick it through next door

Next door will either hand deliver it to them and they will know they have read it or they will just seal it and post next door
