Mix continues to bill my account even after cancelling the service.

I sold my vehicle on April 21st, and have contacted this business on the same day in order the service. I received confirmation of cancellation 2 months later (1st indication of terrible service). Every month since, I have received an invoice for a service that I have cancelled. I have at multiple occasions sent emails and contacted phone numbers that were made available, however fall on deaf ears every time. This is terrible.