Non existing service

MTN has been deducting an amount from my account the last 4 months which was not approved by me. They take my monthly contract plus a random amount. This month it was R450.00. I have been sending emails, phoning, but the deductions keep on going. My profile is now on bad thanks to MTN, since I have to ask my bank to reverse these amounts every month since MTN service is non existing. very disappointing at the service they are given.