Extra data charged without consent and unauthorised debit order

Extremely disappointed with MTN!! I have been charged for extra data which i did not sign up for on my contract. I have been paying R695 consistently for the past 20 months. All of a sudden they charge me R1190 saying i used up extra data. I never agreed to this and now they starting to behave like ******. I would assume a contract is a binding legal document which cannot be unilaterally changed by one party without the consent of the other party. What they are doing is a violation of the contract i signed with them and is really disappointing to say the least. This is **************** at its best! They cannot charge almost double the amount stipulated on the contract. Show me where I have you consent to give me extra data!! I need proof! Please give me back my money because money because you are now acting like ******!! This is really appauling and cannot be right.