Mtn is a ****

I had a contract with mtn for 2years, after that 2years I went mtn store to cancel my contract they told me I should call customer care I did call and request to cancel. But now mtn is busy telling me that I owe R1 789.16 for something I no longer use and I thought it was canceled. Now they are ruining my life with this. I need help because I call and I even send emails but they won't cancel it my credit records is bad because of mtn. Mtn is a ****