You could try banning those search engine spiders from the forums only, and then remove the banning for a few days each month - just to keep the rest of the site in Yahoo, and yet slow down the hammering on your server. It may even get the forum posts in Yahoo if the occasional spider gets through.

I don't know how good Yahoo is with explaining themselves. I don't know that they care that much. I know that DMOZ has forums where people can ask questions about that directory's policies. I don't know that Yahoo has any similar mechanism.