Bad Service from Car Tracker

My experience with Car Tracker has not been good. I have cancelled my contact with Car tracker and paid all the amount that they claimed I owed and advised them to cancel as I do not owe the car anymore- till today they still try to debit my account- send me emails which I feel it’s harassment. I told them to stop debiting from my acc as I can’t keep reversing the amount but the behaviour doesn’t stop!