I had a collection agency after me once - for a charge I'd paid up front and by credit card. They likely knew I didn't owe the money but were undeterred. They harrassed me for years and until I harrassed them back.
I kept calling the person who called me until they disconnected that line. Then I called them on other lines, over and over as they kept hanging up on me. The last guy I kept calling was completely flustered. They stopped calling.
There are sleazeballs flying around in private jets that got them buying debt for pennies on a dollar. Think about this, there are obviously many ways debt can be bad, I can confirm that some of it is "bad" because it IS NOT OWED. We got calls for literally years from from such an outfit, and no amount of reasoning, letters etc. could stop it. Finally out of desparation I began engaging in obscene and lewd conversations with any woman who would call, and depicting male callers mothers and wives in the most abusive and graphic scenarios. It worked after they ran out of people willing to call our number.