I REGRET CHOOSING MTN

I have a contract phone and data with mtn, in December I paid two installment. I was not aware that they already made the debit of which they don't want to refund me. On March I went there to cancel data contract, My phone contract is R849 to add data I have been paying R1700. Since March till to date they are making debit, its either R1700 or R1500. I don't know how many times did I go to their store to complain and I don't get valid reason as to why they don't cancel. Mind you its a prepaid contract data which can be canceled at any time.