Don't think you can. Private messaging can't be removed . You might write to Kass and ask him but as far as I know, they can't be removed. It's blurtit.com. You can go to the bottom of the page where it says "contact " and write to them from there.
How to delet messages on Blurtit.net?
I don't think you can at the moment. You can go to the contact section and ask the relevant admin team member to remove it for you permanently.