Threats for a 'unpaid' account

I had a credit card with you people and CLOSED it 6 or 7 years ago. Now I am receiving messages and phone calls on my work number, there is a outstanding amount and was handed over to collectors?? I phoned Nedbank and the consultant confirmed it was settled AND closed, I need to go to a branch and get a closed up letter they can't email it, SO NOW IT IS MY PROBLEM. I phoned your collector people already twice this week, requesting a letter confirming I owe anything, but these letters also take 5 work days to generate? Stop phoning me, and send me a email confirming my account is paid in full and CLOSED - Ref with the collectors ***4