Mtn makes it impossible to terminate contracts

Last month I called 135 to cancel my contract, they kept on dropping my call, I went into the local store and I sent to century city branch. I canceled my contract. I received an sms notifying me of being on pay as you go, two days latter I received the airtime and data bundles like other months. And they deducted more than the usual on enquiry I'm told that I exceeded the set limit. How can I do that if my limit was set on 0. They just make it impossible to come out of contract they're very inconsiderate as one cancel the contract based on what they can afford and what they cannot