A good post comparing the reaction of the adsense team vs the yahoo team after he reported a total of 13 sites. 5 of the sites reported to Adsense were banned and 0 out of 8 of YPN have not been punished. Yahoo may still be relativly new but they should take a really strong stance against TOS violations.
I think his last paragraph sums it up well;
Get your shit together Yahoo, you’re trying to compete with Google and they’re thrashing you not just in market but in network integrity and traffic quality. Until you actually give a fuck about publishers abusing their accounts you have no chance against AdSense.
There are quite alot of posts in his Blog about YPN and reporting clicks, I would check it out http://www.moddedup.com/