3 months of incorrect

For 3 months MTN has incorrectly billed me therefore breaching the contract we have. I fail to understand how they can continue to ***** from my account (yes, it's theft because this is not what agreed on). At this point in time I would like to cancel my contract and please note I will be reversing this incorrect debit from my account. I will also not be paying any cancellation fee as you have continued to breach a contract we have and think you can get away with it. Your staff keep telling me it's resolved but let's be honest 3months later and your damn system still can't get the calculations right? This is absolutely pathetic that I have to chase after you people to do your damn jobs properly!