I know, it's a joke. I got taken by one for a small amount last month. I've taken to canceling any bids placed by private feedback bidders provided I catch them in time and then blocking them. That's the best I can do.
I called
ebay help, they said the same thing, no preset block for those with private feedback. Since I don't sell often on
ebay, it's easier for me to keep on eye out.