Contract amount debited from my account never the actual contract amount agreed upon

This one star rating is more than Mtn should get. I have the cellphone contract for 4 months now, non of which the actual contract agreement amount has been debited from my account. Each month is a different amount and after sending an email they said its for international sms’s and once the data has been depleted, I’ve never sent international sms’s and signed a contract for 500mb of data and said that if the data is finished I will need to buy again. I want to cancelt his contract is there a way to do this? I don’t want to continue with this “excellent” service provider