Continual harassment over invalid, prescribed debt

I have been subjected to regulars harassing calls (3 today on a Public Holiday) and sms messages (11 in the last month alone) regarding a supposed debt that is not only false but also prescribed. The supposed debt arose when Virgin Mobile tried to claim moneys they were not entitled to when a contract was cancelled in 2011. I told them at the time that I was prepared to let the courts deal with the matter and would be happy to take it in front of a magistrate. Not surprisingly they chose not to pursue their claim for these invalid charges. MBD have no such qualms and have continued to harass me with calls and sms's. It's not even possible to block these as the originating number is different each time. The final straw today was when the MBD representative told me that this debt does not fall under the prescription act and that I \must pay now". She refused to explain why she thought that the law somehow doesn't apply. I do not appreciate the continual harassment over a \""debt\"" that is both invalid and prescribed. As a law firm you should know that you are acting in contravention of a number of laws."""