How about a making up an A5 card and ask the custy to stick it on their front door. Or on a placard in the front garden!

It might say:-
To Scabby Gloster Carrotcrunching Window Smearers (or put their real name

)
I have informed your owner (Mr. Michael Mouse) that I have cancelled my window cleaning agreement with him and his
bunch of monkeys company.
If you do trespass upon my property to clean my windows despite my informing you not to then you will not be paid. If you attempt to collect money and become intimidating about it when I tell you to leave then I will inform the police that you are demanding money with menaces.
Yours sincerely
Mrs. Lady
Custy
Or if your custy is elderly then type up the following kind of letter for her to sign and post.
Dear Scabby Gloster etc,
Re Cancelling of Window Cleaning at 25 Cromwell St.My sons are church going antique lead-roofing recyclers who enjoy travelling the country in their lovely shiney mobile homes and flatbed trucks and when in this area they call to see me for a cuppa and a chat on the way to the scrapyard to sell their latest acquisitions of lead roofing.
If you like I can send them round to your gaff so that they can explain in person that your services are no longer required. They may ask you to join in a friendly game of baseball to ensure you get the message.
Yours truly,
F West (Mrs.)