MTN charging me after my contract is cancelled for over a year

I have cancelled my contract over a year ago with MTN. I have called them and have proof in an email that they have sent that I don't owe them anything. Till today i get sms's from them saying that i owe them money and that they have handed me over to Nudebt. This is ridiculous. Its over a year now. *********** and unprofessional MTN as usual.