Yup, agree with Dazmond and Sean. I have one in particular who owes me 90 euros for several cleans. I haven't written it off, but if she ever asks me to clean her windows again (and she has, believe it or not) I remind her of her debt and once she has settled that and paid for the fresh clean up front, I will do them for her.
It actually makes me laugh now. I know I'll never see the money, but then she knows she'll never have me over again.
John