Unfair billing and blacklist

In 2012 I took a cell phone contract with Cell C. Which was due to end in December 2013. In September 2013 I approached Cell C in Cape Gate where I took the contract to request them to allows me to buy myself out of the contract. They refused saying I am not allowed to. I the requested them to then close any star costs on my phone that exceeded the contract amount of R300 so that I can only pay the agreed upon amount. They assured me that they will put a stop to any costs and keep me on the set amount until the end of the contract. <br> <br> Surprisingly in November they billed me an amount exceeding R4000 and I questioned them where I came from after I had been to them and they assured me that there will be no services beyond R300. They responded by saying they placed a stop on the calls not on the Internet. <br> <br> I was not aware that I had open Internet or they did not place a stop for the Internet as they said there will be no extra services and therefore no extra costs. <br> <br> I have just learnt that they have blacklisted me at credit bureau as I tried to open an account and I was informed that my name is blacklisted. <br> <br> Please assist.