Calls Harrazment

I have a prescribed debt with Capitec and even on my credit report the debt is closed. Now MBD says they bought the debt and they are harassing me with their calls. I denied the debt and their consultants are rude and insulting, they are calling my work place and my cellphone like crazy. I asked them nicely not to call my work place as I could be fire and because we are not allowed to get personal calls at work but to no avail. What must I do to stop this crazy calling.