Contract Cancellation

My mother, a pensioner, has been battling to get her data contract cancelled for 4 months now. Every month we phone to cancel only to find the debit order going through on the first of the following month. Every time we phone, we are promised that the contract is cancelled but come month end, the R155 gets deducted. There is a reason why we are trying to cancel the contract!! She is a pensioner and the contract has ended, we do not need it anymore, we did not renew the contract!!!