Inconveniencing me costing me money

I took out a 5 gig data contract with Vodaciom. They gave me the data the first month which was in mid November. However they double debited my account as i am in a credit balance with them of just under 500 rand. No problem i can live with that. Secondly we get into the new month 15 days into my contract, they not only not lot load my data but also take away the 2 gigs of data remaining which supposed to carry over to the next month. So now my airtime that i loaded to make calls from is being deducted for data use. This is the worst service ever. I don't want this contract. I can pay cash for the modem provided with the contract but i am not prepaid to pay any extra cancelling this contract for sloppy service. This is totally unacceptable. I have been loyal to Vodacom for years but i don't want this contract anymore. I am still waiting for Andries supervisor from cancellations to call me back.