With Adsense there a is way to report extreme abuses by adsense publishers. You simply click “ads by google” in the ad block, and then click the link that says “Send Google your thoughts on the ads you just saw”. You can also send an email to adsense abuse.
I haven’t seen an equivalent option for yahoo ads. Is there a way to report extreme violations by YPN publishers (such as disabling the back button so users have no choice but to click an advertiser's ad, thus cheating advertisers out of their ad dollars)?
If you are a YPN publisher, just send them an email. You know the address. I've heard that they are VERY responsive. Like the offending site's ads were down in 20 minutes. But that was a more severe violation...
Anyway, I'm sure that Yahoo will listen to your email and take action. They are really good at this, unlike Google. ;)