You should contact the merchant in writing and tell them to cancel both the service and the charges to your account. You could consider including in the letter information on how you previously requested to cancel the service or provide a copy of any prior cancellation notice. It's a good idea to keep a copy of the letters you send to the merchant for your records.
You also could contact your bank and tell the bank that you are revoking any authorization for these merchant charges. However, you should still contact the merchant to cancel the service. Consider including information to help the bank identify the charges, such as the name of the merchant, your account number with the merchant, and the amount(s) and date(s) of the charges.
Another possible step to consider is to ask your bank to place a stop payment on a pre-authorized transaction. To stop payment, you need to notify your bank at least three business days before the transaction is scheduled to be made and your bank may charge a fee. The notice to stop the transaction may be made orally or in writing. A bank can require written confirmation of an oral stop payment request. If that written confirmation is not received within 14 days of the oral notice, the oral stop payment order ceases to be effective.