Another unauthorised charge

For a 2nd conscutive month Telkom has added an unauthorised charge to my invoice. This was once again not requested nor authorised by me. This contract has already expired and is suppose to be R99 on a month to month basis but instead is being charged R456.50. I contacted Telkom in this regard last week but has not yet received any reply. I want the charges to be reversed before the next debit order date and don't want a situation like last month where I had to pay the extra unauthoried amount and then to be reversed on the next invoice. I also want a guarentee from Telkom that this charge will NEVER be added again to any of my future invoices.