For what reasons can Yahoo! terminate your Yahoo! Publisher Network (YPN) account?
The answer is for any reason they want.
According to the terms and conditions Yahoo (aka Overture) has the right to terminate your account for any reason they deem necessary.
We may suspend or terminate our provision of Matched Ads to you at any time, with or without notice, for any reason or no reason, with or without regard for how well Your Site or Your RSS Feed is performing, the quality of user traffic coming from Your Site or Your RSS Feed, or any other factor, in Overture's sole discretion.
Side note: I am curious why the legal document is still from Overture, aren't they going to legally change everything over to Yahoo! eventually? :)
But you as a publisher can also terminate for any reason you want.
You may terminate any of Your Sites' or Your RSS Feeds' participation in the Beta Program at any time by removing the Ad Code from that web site or RSS feed. Upon your termination, we may but do not have to immediately stop providing Matched Ads to Your Site or Your RSS Feed. If you place the Ad Code back on Your Site or Your RSS Feed, you will be subject to the then-current Terms and Policies.
Can you be terminated for not running the ads on your site? That is the question a DigitalPoint Forums asks. Technically, yes - but highly doubtful.
As one member points out, you can also technically be terminated if "you sneeze and cover your mouth with your left hand."
Keep in mind, Yahoo! terminates accounts if they deem necessary. They typically do this if they feel the publisher is exploiting their advertisers in some sense. Yahoo has done a lot to keep their publishers aware of potential violations through their revolutionary compliance manager section.
Forum discussion at DigitalPoint Forums.