Yahoo's domain blocking tool is built to block ads based on what site they appear on. So if you set it to block ads on ideo.net, you won't see them on that site.
The problem is when ideo.net gives their feed to every domain in their system. So xyz.com signs up and adds the code to their site. The Yahoo system sees that ad as being shown on xyz.com, and not on ideo.net. Therefore, just because you blocked ideo.net doesn't mean you blocked all the millions of sites they've given their feed out to.
But here's the rub. xyz.com has all their ads redirect through ideo.net. Therefore in your logs, you see the traffic coming from ideo.net and not xyz.com. It makes it impossible for you to block the site or tell where any of this traffic is coming from. It's an unfortunate loophole that has been there for years now.
Unfortunately, Yahoo isn't really search marketing anymore. It's parked domain marketing. That is where a good percent of their traffic comes from these days. Traffic that can't really be blocked. Google seperated this type of traffic and allowed users to block it. Yahoo currently doesn't want to give you that ability for obvious reasons. All I can suggest is sending in a suggestion to them about it, although I'm sure they've heard it a million times already.