Talk about lack of targeting, eh?
I'd say you've handled it okay so far. I doubt seriously if I would have done as well. I'd have been highly pissed to say the least. As for what you can do now you can only wait. Or you can file suit. Unless you want to press the litigation button you're basically at their mercy. They've told you they're investigating. You have to assume they are and you have to hope that somebody over there with a sense of fair play will be the one responsible for this investigation.
Sites with no content are in violation of YPN's own terms. If clicks from those sites sent traffic to your site you should at a minimum be reimbursed for whatever those clicks cost you. They should also pay you for the time and effort it took you to bring this to their attention. You'll never see that last but in the spirit of fair play you should.
You have a strong case though. Good luck with it.