CONTRACT CANCELATION IS A **** WITH MTN

I am not happy with MTN SOUTH AFRICA THEY ARE A BUNCH OF ****MERS ,I want to cancel my contract with them I had a balance of R1200 which I paid and I called in to cancel the lady who was assisting me then offered me an upgrade of which I was ready to take it ,there after the process was done and later on I changed my mind that I no longer want a contact let me rather cancel ,when I called in they say that I have an overdue of R499 what the F ? How as I called in they told me that everything was fine now all of the sudden I have an overdue ?? I told them to stop charging me they told me I have to wait for 30 days for my cont6to be canceled and also they would again invoice me during that process ******* how the hell does that happen when one is requesting for cancelation at you still have to pay on top of that ??? I need urgent assistance in this matter because I can't go on like this my contract has ended i am struggling to cancel this ass h...l contract