Contract expired and are still being charged monthly

I went to Cresta Cell C to have my contract cancelled a month before it end. All was done on the system and told them that we do not require any further service from Cell C. It is 3 months later and yet a monthly deduction go of my bank account. I require my money back and all further deductions to stop. I did everything from my side but surely Cell C have a problem with money that must be why they keep on deducting money from my bank account.