Continuous overcharging on data account

In February 2015 I received an invoice for my Broadband 2GB@R99 account. It was for R6618 an overcharge of R6518, my average for the previous 12 months was R99. After emails and calls to customer care I eventually got a credit for R4313, R2205 short, which was not paid back into my bank account but crediting my MTN account instead. In the process of trying to get my money back they start it again don't they! June, July, August, September, overcharges of R2148,R6067,R1721 & R4049 respectively. This adds up to R16515 of overcharging '& emails & phone calls are totally ignored with only a promise of \we are looking into it"Never give this outfit your bank details they can only find it to take money. They don't where it is when they owe money!"""