Over billing my account

I was called by one of the Cell C consultants this year in August and they offered me a contract ( AG Mobile tablet for R150 a month). I took the contract and agreed on paying R150 per month. Unfortunately Cell C did not stick to their agreement as the charged me R200 monthly ( They pull the money electronically). I went to a cell c store in vaal mall and they told me a different story. Now I have noticed that the amount they are pulling is over R200. I am not happy at all and I am going to make sure that when month end comes, I am going to withdraw all the money so that they wont be able to pull even a cent from my account since they are not sticking to the initial contract I signed for.