Cancellation nightmare

I canceled my contract with tracker because I sold the vehicle that had this device. They debited my account end of May with the cancellation fee, yet I am still getting invoices and debit from them!!! Why don't you just cancel this damn thing and leave me the hell alone!!!! Stop debiting my account and do your job properly by removing my detail?? How the hell do you debit the cancellation fee then you still come afterwards to debit a monthly fee???!!!