Why is it difficult to cancel contract with mtn

I have never had a bad service provider than mtn, the worst i have ever experienced. I canceled my contract in September but the billing still continues this day. I called many times and all I get is reference numbers I don't know what to do with them because they are useless. They suspected my line 4 times already and now they don't want to open it, I am told they are still busy with investigation. I have nothing to do with your investigation, but why close my line. Why is it difficult to cancel contract with mtn