Harassment of a paid up account

I was on contract, i fully paid my account then the converted me to prepaid... After about a year and a few months i started receiving emails And sms's... I called the call centre they to me to go to the branch... I went to mtn in Randburg they to me it was possible ***** and yo ignore them... The emails and sms's continued i went to mtn in cresta they told me its impossible i still owe because mtn would have never converted my account if it was still owing... All of a sudden im handed over now im being harassed with calls 5 times a day... FIX THIS I DO NOT OWE MTN A CENT... THINKING OF TAKING THIS FURTHER LEGALLY