meta name="robots" content="noindex, nofollow"
But yahoo doens't look at your metas, its a hand reviewed directory.
You'll probably have to go to the place where you would submit a site to yahoo and look for a link to have it removed, or email the chief editor or something like that.
But you know he could just leave it, and if he decides to become a porn star one day it would save hime $600
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Did somebody say Free Content?