Bad service, unauthorized debit order in clients accounts

Cell C has debited me twice last month and I don't know what for. I have been trying to communicate with them and they keep on saying I reversed the July debit order if which I did not. I only reversed one of double debit orders last month. I sent bank statements and no one bothered to call me back. Today they debited in my account without my permission and there are no funds now they causing me unnecessary charges and negatives in chequue account. I'm very angry if its possible I want to cancel my contract with them. Their customer service is bad. I'm so angry and I will never renew a contract with them.