Bad service from telkom

I had a contract with telkom which has ended last year and had send cancellation letter last year september,but they are busy deducting money from my account every month.I have been calling them since then they i'll tell me to send in that cancellation form each and every time i call,but till today my issue is not resolved.I decided to do a reversal of payment every month,but they continue to debit my account.Now they send me a bill Due by 31 Mar 2018 R 597.88 acc: ********** 23 and cellphone in question ********** 770.Please help