Standard Bank
Loan paid off, still receiving "threats"

I had an overdraft facility with Standard Bank that I paid off months ago, just to receive sms' from MBD inc. that I still ow on the account. I went in to the branch and even after realizing that it was banking fees (even though I requested the account to be closed when I paid off the debt previously) I still made the payment (again) and closed the account.
Now I still receive threatening sms', i.e. "YOU HAVE NOT RESPONDED TO PRIOR MSGS ON YOUR STD BANK LOAN ACC. PAYMENT IS REQUESTED WITHIN 48 HOURS, "
I have phoned them in the past and explained that I did close the account and in which branch I closed it. Surely it is their responsibility to check whether I closed it and not mine to forward the proof (ESPECIALLY AS I CLOSED IT IN THE BANK).
They do have my email address and I have never received any emails from them with a proper statement or anything, only sms'.