Yahoo appears to be releasing a new search marketing feature that enables advertisers to block specific sites from displaying on both Sponsored Search and Content Match networks.
Discovery at the Search Engine Watch Forums thread writes that the "Site Blocking" feature should be available anything between now and the first quarter of next year.
Yahoo Site Blocking will enable you to prevent your ads from displaying on a specific domain or sub-section of domain at the account level. Yahoo will allow up to 250 blocked domains, while Google only allows 100.
You are able to configure blocking a entire domain, sub-domain and up to two directories.
Forum discussion at Search Engine Watch Forums.