No solution from tracking company

I sold my car and canceled my subscription for tracker a few months ago as I no longer have the vehicle the company mix telematics sent me confirmation letter that it was canceled but they keep billing me every month. I get a statement every month saying this is my installment for the current month and when I reply showing them the cancellation note they give the same automated reply of case number and they looking into it but nothing happens and end of the month same story they send statement now they even say they will report me and I will get bad credit rating.... It's very frustrating.