Exorbitant Charges on Accounts

I have an account, with two cell numbers, for my kids. These packages was always top-up packages to limit the risk of going over the amounts of the contract/charges agreed too. To my surprise I see massive deductions from my account for these telephone numbers and when asking the question (to mtn customer service) the answer is returned that it is data used....and that there is a R9000 credit limit on the account. I asked for the contract where I signed for this credit limit, and asked for MTN to stop/cap the numbers to prevent the overrun of contract fee. This is quite a important matter to me as there are R6000 deducted over three months from my account. I went to a store today to assist me (because you get no response from customer care) and they could not assist with capping the expenditure for these numbers. It seems that Mtn only responds to complaints lodged on this website, I am curious to see the reply and how do they justify providing credit to anyone without consent. Is that not in contradiction with Financial Services acts?