complaint - contract

I took this top up tablet contract out on 6 june 2015 and was billed for an open line up to today. I queried it with the branch on 21 november 2015 and to date after spending on numerous calls with mtn they kept telling me that my account is in credit but normal debit order is going off. I refuse to pay this month due to your staff being incompent and should be fired. Warren Potgieter - cell number *** loaded my query and according to the call centre two weeks after that made an error with my cell number and was not logged. Ive had it with this ****