MTN statement

I have cancelled my contract with MTN a multiple times and I told them I am currently unemployed, I fully paid my contract but to this day I am getting emails of my statement, threatening that if I don't pay I will get blacklisted and that I owe them for their services but I had asked them to change my sim to prepaid .when I call their agents I'm am told that I do not owe anything and my contract was canceled and the sim card was changed to prepaid, I'm really feeling helpless and I ask them to please sort this out because I am tired of receiving emails from them.