Poor service

I had a contact with mtn about 3 years ago, upon paying it in full I went to the branch to cancel it since I was done paying for the phone. For some reason the contact was not cancelled and got handed over to the CSS group. I then paid the outstanding amount to settle the contract. I made sure that I send the prove of payment, however till this day I still get emails and sms’ stating that I need to make urgent payment. It’s really unfair. I really need mtn to sort out the issue with CSS.