Misleading Treatment of billing package

I purchased a 5mb data wifi package from vodacom, and after 2 months of excessive ot of bundle charges I contacted vodacom, and they informed me that it was because I had put no cap on my data package. At that stage I informed them that I did not want any more than the 5mb of data I had purchased each month, for which I was paying around R250 per month. I was then informed that I could put a cap of about R50 over and above this, after which it would stop. I confirmed that this would be fine as I do not want any further bills over this amount. The next mth I received a data bill of R800 over and above the R250 monthly charge, when I phoned to complain they informed me that this was for data prior to the implimentation of the cap, but with the cap \IT WONT HAPPEN AGAIN". I confirmed that I was happy with this as I cannot afford these excessive bills. Last mth I received another bill of R800 in excess of my R800"