Non cancellation of account

My contract ended in December 2015, when I called the call centre they told me to call one day after my contract ended. By the time that date came around I was not in the country and only came back in January, I called in to have the contract cancelled and changed over to prepaid on the 13/01/2016. The call centre agent promised me that I would only be debited in January because I called in on the 13th. I asked her Three times to make sure that this would be done. On the 20/02/2016 my account was debited and when I called in today (22/02/2016) , I am told that the request was put through but not followed. I asked them why their retention's department was not doing their job as this should have been sorted out in January already. I want this sorted out IMMEDIATELY or I will take this further. It is ********* to be dishonest to your customers. I must say MTN is the worst network I have ever come across. This is not the first time I experience such disgusting service from them.