Harassment through calls and smses

MBD Collections have been calling me about Standard Bank account claiming that I owe. I closed the account in Essselen Street Standard Bank branch in PTA, Sunnyside in 1999. I used to have R500.00 overdraft on the account but was 00.00 balance when I closed it. I went there personally to close it. They are telling me that I owe R5000.00 for the same account and must it to them. I closed that account personally at the branch and owed nothing at that time. PLEASE STOP SENDING SMSES and YOUR PEOPLE CALLING ME WHERE SOME EVEN SWEAR AT ME AND CALL ME NAMES.