Utter Incompetence & False Legal Action

I closed my Standard Bank years ago, possibly 2007 or 2008. I opened an FNB account and transferred all my funds from Standard Bank to FNB and then closed the account (Ferndale Branch if i remember correctly). A few months later I received a bill from Std Bank for bank fees, I went back to the bank and told them the account was closed, they apologized and said they would correct it. The same thing happened a few months later, I then went to a Benoni branch (Lakeside Mall) and they also apologized and said they would fix it. This happened about 4 times! I didn't hear from Std Bank for years until this year where debt collectors are now saying i owe Std Bank money. I have told them over and over again that the account was closed please speak to Std Bank, they have now resorted to LEGAL ACTION! I don't know what to do anymore? Their incompetence and lack of communication within their institution has led to all this unnecessary harassment and a waste of time and money