Unauthorised billing of account

Tracker continues to debit my account three months after I requested them to cancel the contract which I no longer required after I sold the car which had their devide. This issue dates back to November 2017. Upon the expiry of my 36 Month contract agreement with them, I requested tracker to put me on a month to month basis following endless calls from their consultants who were trying to sell me more products. I declined their offers citing that I would soon be upgrading a car and it would be pointless to renew a tracker contract given those circumstances. Now I cant seem to get them to stop debiting my account for a car I no longer have in my possession. Its ridiculous.