Contract Cancellation

My Mtn contract expired in June this year. I've been trying to cancel it since September but Mtn has failed to cancel it. They are still deducting from my account and I have called their call center numerous times to try and rectify the issue. Still nothing! When I stop paying them, they suspend my line. I then have to explain my problem and they fix it temporarily. I don't understand why they are wasting my time and trying to get money for something I cancelled.