CONTRACT CANCELLED BUT STILL BEING BILLED?

I'm very upset with the service I am receiving from Cell C. I did a pre-cancellation of my cell c account and have called on several occasions to ensure that the amount paid was for CANCELLATION. I have since been incorrectly debited twice and its the amounts I have paid, which makes no sense at all! I had to go to the bank to reverse this. I went to the cell store afterwards to sort this out and they phoned cell c. Cell c confirmed to him over the phone that I have nothing outstanding and they would stop the debit order and nothing would be deducted agian. On 15 September the same amounts was debited again and this now costs me to go back to the bank to return it!!! This is ridiculous. Not only is this costing me having to go back to reverse it, its also costing me additional bank charges as I should not be debited for this amounts. I expect cell c to contact me urgently and stop this debit order with immediate effect and also refund me of all the bank charges as this is Cell C's fault! not mine.