Unauth'orised credit for R36000.00 on bill for data

No one in MTN can explain how did I get (credit)and invoice of R36000.00 for data used in a time frame of 2 months without my knowledge or consent. Is the credit act not there to protect the public against unrealistic or un-affordable credit? Why is my credit limit on there computer which is <R3000.00 not used to stop me from excess credit or data use? No one in MTN is willing to accept responsibility in solving this case. It is ongoing for 1 year now. All they do is to switch off all my cell phones and forces me to make a payment which I can not afford.