Cancelled contract at end date, billing continued for months afterwards

Had a contract ending in January 2017. I contacted MTN and formally cancelled the contract at the beginning of January for which I was given a ref, but also told that I have to phone once the contract actually ends. I repeated the process early in Feb for which I have another ref. The billing continued after each month end. In April I went to an MTN shop to rectify this, they sent an email with me on copy to ensure that this is sorted out. I continued to receive bills, so I reversed them with my bank. I now have attorneys phoning me from a collection agency! I have explained my lot, provided the refs, and a copy of the email, and MTN just respond by sending me a bill with a breakdown of how much I owe them..... They say they cannot find those refs. I refuse to be bul**** into paying amounts I do not owe. Furthermore prepaid airtime I have purchased has been accepted, but it cannot be used. I guess the SIM is disabled now as well. My advice to anyone reading this, do NOT enter into any contracts with this provider, when I spoke to my bank about reversing these deductions, they said they are having a lot of problems with MTN, so I am clearly not the only one.