MTN unresponsive to billing dispute application

In January 2015 i upgraded my contract, and for the first couple of months all went well, my account never went over R1000 per month. however, the last three months i've been billed ridiculously large amounts for data usage (this is according to the itemised billing) even though i've been purchasing increasingly more data bundles. I can not afford to pay R2000-R4000 a month. Upon request I am only told that I need to set a limit on my open line once my free or included data and minutes have been used up. Although i've used up my free minutes the *141*0# service still tells me that i have data available, however, i'm getting billed at the normal data rates. I have indicated that i want to formally dispute my bills and MTN needs to investigate, and have been advised telephonically by customer care that they would forward me the documents, but have received nothing. i have also emailed customer care but have received nothing.