MTN is the worst network provider

MTN keeps charging me interest for a contract that ended a year ago. When my wifi the contract came to an end MTN did not automatically cancel it. Instead they began charging me double the amount I was paying on a monthly basis. At this point I was apparently owing them the amount of "R1600" I personally went to one of their stores to settle that amount even though I didn't understand how it came to that. I then asked the guy who was assisting me to please close that account and I told him that I want nothing to do with MTN ! He told me that the systems was offline and that I should call. I did as I was told and the call center agent told me that the contract had been closed successfully. Now it is almost 18 months later I am still owing them and I am being told MTN is showing up negatively on my record. I do not understand this! Everyone aroused seems to have the complaint when it comes to MTN. Why do they make money by false fully overcharging their clients