Billing of a Closed and Paid up Account!!

I had accacount with Vodacom I*** and paid the balance amount in January 2015. The account was handed over to Vodacom's collectors VVM, who have been sending me emails and sms's demanding payment. I have called VVM to explain the situation, email them the invoices. I have also emailed Vodacom legal and sent the invoice as proof of payment, still the threat emails and sms's continue demanding monies I dont owe.<br> <br> Fix this matter urgently.