Vodacom is useless

My contract was coming to an end so i phoned Vodacom to cancel it as i did not want to renew it, but after it ended they kept billing me. Eventually after 4 months they stopped but it has left my account in arrears wich they expect me to pay. I phoned customer service and they said they will look into. My vodacom account is still in arrears and now i have been billed for Airtime Subsciption charges which i have never used.