Keep on deducting money

My contracts with Cell C was cancelled due to there mistake made. They told me that both these contracts will be written off and all payments due cancelled. They do not keep there word and continue to deduct money from my account every month trying to get payment out of me. I have to reverse these payments every month. If I do not watch my account like a hawk they might succeed in getting unauthorised money out of my account. Various attempts to stop this counts to nothing.