Cancelled contract stil being debited

I cancelled my data line with MTN on 31st August 2015, the call centre agent had told me earlier that month that I can only do it on the last day that the contract was valid which was the 31st so that's what I did. MTN then debited my account on the 1st of September, I left it thinking that would be the last time they would debit my account. On the 1st of October they debited my account again. I then called the called the call centre and emailed the customer service department, gave them the persons name I spoke to on the 31st of August as well as the reference number. MTN then again on the 1st of November debited my account and again I emailed and was told that I had to cancel within 30 days which I then explained I did and again gave the reference number and also reminding them that the contract also expired. I received an email back to say that they can cancel but I have to send through proof that I have been paying it. Why must I send them proof, do they not have a record. To make matters worse I received an account that they will be debiting my account on the 1st of December again for the contract that I cancelled. I do not know what to do anymore. How do I get my money back.