Cell C Account not Closed

I am very Irate because I paid off an outstanding with Cell C : Acc No-***9 in the year 2010. I received a document stating that it is paid in full and that the account is officially closed stamped by the Cell C head office dept. But I have constant calls every year stating that my account is outstanding and I need to make payment. And everytime I send them a fax or email showing my document that clearly shows that the account is fully paid off. I immediately get an apology from the consultant stating that my account has been fully paid and they will definitely check whats happening with the account and close it. But every year I get a new consultant calling me saying that my account is in errors and I need to make payment. It has been 5 years of this and I want it to be rectified. Nobody seems to be sorting this problem out and now I am angry as I do not want this to go on for another year!!!