Wrong debit amount

MTN started debiting a new amount on my account , which is double the amount i signed up for, they claim that i suddenly changed my data usage to no limit, and now they charging me a ridiculous amount, when i ask for prrof and a contract for that, they cannot provide it, my contract was a top up contract which means that once i have used my data and minutes i recharge it on my own, hqve been doing so for q whole good year now they come with this nonsense, this costs me bank charges because i have to reverse the wrong amount and call them to deduct the right amount, i am fed up with such service, last month i gave them a chance to correct this and they did it again this month.