I am regretting why i contracted myself with Vodacom

I am currently contracted with Vodacom for monthly installment of R750 . Last week i have received a call from Vodacom saying, they discovered that i am using too much data on my phone and they are going get me into better data usage without any additional payment. come to my surprise when i check the statement the amount of R750 went up to R999.99. I phoned Vodacom and they didn't explain anything to me and they put me through to the other department only to find out that, that department is 3rd party ( SO MY INFORMATION WAS GIVEN TO OTHER PEOPLE WITHOUT MY PERMISSION WHERE THE PRIVACY OF CLIENTS INFO) During the call i keep up on asking does this upgrading costing me anything and the lady keep on saying no. i always complain about the usage of data and airtime since its abnormal. i would mind if i can get legal authority to cancel this contract with any payment because am seek and tired of Vodacom