Pathetic service and incorrect billing account

I have a Cell C data contracts with a monthly fee of R99.00 each. When I signed the contract, i was told that should I deplete my 2 Gig data for the month before month end, I would then have to buy more data which is what I signed for. In this way my fees would never exceed R99,00 regardless of my usage of internet. I don't even use the sim cards to make calls. To my suprise, in December I was suddenly charged R2477,97. I then went to the Rosebank branch to get clarity, the consultant was also surprise as to how the figure was calculated. She then called Cell C head office so that they can explain and I was taken from 1 department to the other until they cut off my call. How did this happen when there was a data limit on the account and was informed that the limit was active but they could not explain why it was exceeded. Since then, in trying to resolve this issue, I have been tossed from pillar to post. Today, my contract are coming to an end and i want to cancel both of them. I'm so fed up with Cell C, i spent 80% of the time without signal and yet they have the audacity to overcharge me. Rarely have I seen such atrocious service.