Wrong billing

My wife took out a contract with MTN for a tablet and 500mb data. The contract is supposed to be R328 per month excl. VAT and insurance. We do not use the 500mb data per month but get billed R1434 plus per month. How is that possible? We are not getting any help from the manager at Selcourt Towers at all. She says that she has registered the complaint with head office but that nothing is being done about it as they are striking at MTN's head office. What now? Should I change my banking account and let MTN try and recover the money from me or what?