Netstar shocking - refuse to cancel a car tracker debit order

We sent written instruction many times to Netstar to cancel a car tracker that was sold early last year, however the contract has never been cancelled. The debit order keeps coming out off our account every month. We always call the call centre and speak to someone different. We then email a notice to cancel, however it never gets cancelled. When we ask on email for the contact details of a manager or director at Netstar to escalate this to, contact details are never provided ! It appears as if there is deliberate intent siphoning our money every month !