I just sent this message to the Orange mobile phone company through their website
Hello! As you'll notice I've chosen the 'I'm not an Orange customer' option at the top of this form. In fact, I haven't been an Orange customer for over a year. However, I'm really pleased to see that my not being an Orange customer hasn't deterred you from sending me regular quarterly statements saying I owe you £0.00. Thanks for keeping me informed! It's nice to know that, as a non-user of your services, I don't owe you any money. I was already pretty certain that I don't owe you any money - what with not being an Orange customer and all - but it's nice to be reassured. Presumably you send similar letters to the 5.95 billion other people around the world who aren't your customers, to reassure them too?
On the off-chance you would like to stop sending me these statements - saving yourself some money, my postman some effort and our planet some trees - the statements come with the account number xxxxxxxx written on them. I have telephoned you about these statements at least three times before now and have, on each occasion, been assured that my account is definitely definitely definitely closed - definitely! - and I would not get another statement, so I don't know if this number will be of any use to you. I offer it for what it's worth, with the knowledge it might be as random and meaningless as your telephone operators' assurances.
Best wishes, and have a good weekend,