To Whom It May Concern:
I am writing to request that you stop (insert company's name) from making future automatic withdrawals from my checking account. These charges are unauthorized. I canceled my contract in writing with (insert company's name) on (list your date of cancellation).
The Federal Reserve's rules governing pre-authorized transfers (part of Regulation E) states the following: "Once a financial institution has been notified that the customer's authorization is no longer valid, it must block all future payments for the particular debit transmitted by the designated payee-originator."
I would appreciate your immediate compliance with this federal law. Please contact me if you have any further questions.
(Your account number)