Continuous Charges Even After Cancellation

I cancelled my service multiple times OVER A YEAR AGO for a vehicle I NO LONGER OWN because I continue to be charged for it. I should now only have Wifi service connected ($15), and thought the issue was resolved, until I was randomly charged AGAIN for a service plan ($34.99). HOW MANY TIMES DO I NEED TO CALL ONSTAR TO STOP THIS RECURRING CHARGE THAT I DID NOT AUTHORIZE!? I filed a BBB Complaint hoping to resolve this recurring issue. Any insight on how to stop this from continuously happening would be great!