MTN charging me extra without my consent and not cancelling the contract

Worst experience ever.My data contract was due for an upgrade in december.i didn't upgrade but they added promotional data and charged me for it without my consent.i was paying R199 for 20gb and now the price is around R320 for 20gb.Ihave called in numerous times to cancel the contract but their staff is *********** and haven't done so.i am beyond frustrated.I was using the branch in Davenport in durban