unauthorized payments

I have been with Cellc for a year, then all of a sudden there is no cover in my area, why only they will know. I phoned everybody at cellc and they could not resolve the problem they then send me a cancelation nr after I filled in a form the nr SD632002 for canceling my contract. that was in July 2017. Since then they keep on putting d/o throug for R2900, R1000, R900, R1075 . My instalment was R575 and I never missd a payment. Heaven know what these payments are for, surely I am not going to pay for their incompetence!!. My bankk reversed some of these payments, but they are chhanging ref nrs and amounts and keep on putting amounts through. I have send several mails to them but they ignore me like a stopstreet. What is the next thing to do, sue them as this is *****!!