Mtn deducted money after my contract was canceled

My internet contract with mtn was cancelled in March (this year) and I received confirmation from mtn on 16 March that it has been cancelled. There has been no data usage since the end of March (as I was under the impression it was cancelled and I was overseas so could not use the internet) but mtn kept deducting money from my account each month. At the end of April an amount of R299 was deducted, at the end of May another R299 was deducted and at the end of June R314 was deducted. That is a total amount of R912 that was deducted from my account after the contract was cancelled and no data was used. At the end of July I received a refund from mtn for R299. I am still waiting for the rest to be refunded to me. The amount of R613 is still owed to me by mtn. I spoke to a lady called Veronica at mtn head office and gave her the whole history and she said that she would investigate and get back to me. I still have not heard anything. I need feedback regarding my case urgently. I will not let this go and want the rest of the money refunded to me asap.