Harassed for weeks for an account that's not mine!

I have been contacted about a supposedly outstanding amount on my Mogale account on 21 August. When I informed Palesa that I did not have an outstanding account, she insisted that I had, and e mailed me the account. The account was for a property I have never owned or lived in, the name of the person is completely different to mine, as is the account number. I informed her via e mail that this was not my account, and that mine is not in arrears. To date i have sent her 4 e mails, but I have now been harassed for almost TWO MONTHS by Revco, and am consistently receiving threats from them via phone and SMS,. Just this morning I received another SMS from them threatening me with legal action. What else must I do to stop the harrassment???