You can prevent most robots, including Yahoo!'s, from indexing individual pages by placing the following meta tag elements in the HTML of the specific page:

<META NAME="ROBOTS" CONTENT="NOINDEX, NOFOLLOW">

You can specifically block Yahoo! from indexing individual pages by placing the following meta tag elements in the HTML of the specific page:

<META NAME="YahooSeeker" CONTENT="NOINDEX, NOFOLLOW">

Click here for more information on web robots standards and practices.