Harrasment over probable prescribed debt

I am being harassed via SMS and phone calls by MBD Attorneys regarding a debt I know absolutely nothing about. I have emailed requesting details of the account and detailed statement but to no avail. When an operator calls me I request this information to ascertain the validity of their claim but I have never received this information.The harassing SMS's threatening that legal action is imminent with at least 2 calls a week. I have ascertained that this has something to do with Standard Bank and I last dealt with this bank in 2006 so if it is for anything regarding dealings in 2006 then the debt has prescribed anyway so why the harassment? This has been carrying on for over 2 years and I have had enough now.