Prescribed debt

I called african bank to inform them that my debt has prescribed. They came back and told me they can not cancel it as it has not prescribed. They promised to send me a letter so i can lodge a complaint with the bank ombudsmen. It has been two months and i have not recieved anything from them. Everyone seems to not be aware of what am talking about. Yet am being bombarded with calls everyday. Please send me that letter or remove that debt.