Breach of contract

I'm very unhappy with the way Vodacom are doing business. I've got a 24 month contract for R577 a month instalment for my cellular contract.for the past two months they've increased my monthly instalment with a R100 by adding more data .I never asked for this extra data and it was never mentioned in my contract. They are forcing data on me without my permission. At the end of this month ,if the same dishonesty happens I will reverse the installment at the bank and pay my R583.17 incl 1%vat instalment at a Vodacom branch myself. I will also deduct the other months money that vodacom took without my permission. My account number ********** 9-9