Poor service and messing up my.life!!

I am very Angry with MTN ,had account with them 2014 and paid and closed it off 2016 but it's still showing and it's making thing's difficult for me eventhough I have gone to their customer service shops to query they don't get back to me..Now my credit is a mess because of an account I have Paid!!MTN please ,I want to get my life bac****