Repeated harassment

I have a cheque account with standard bank 12 years ago which was never used or transacted in, i never activated the account, standard bank is now every 6months to a year harassing me for over R1000 that i apparently owe them for this matter. I have repeatedly told them i closed this account years ago, i have also spoken to several supervisors and call center people that assured me that this matter is dealt with and closed. Today again i got an insulting rude phone call from some stupid collections agent. I do not appreciated being harassed!!!! sort out this matter