No, we can't tell you that here.
But, basically, it doesn't matter. If you do not have a core of content -- a community of people already discussing something (or already visiting a content-rich site for information about something) -- if you do NOT have this, then it doesn't matter how many forum-spammers find the site through Google: nobody will ever use the forum except the forum spammers; no real visitor will ever stay on the site for a moment after seeing the forum-spam. You really shouldn't be starting a new forum; if you have something to say, you should be visiting some of the forums that are already set up on the subject of your interest.
And ... if you DO already have a community of visitors, then you do not need Google and Yahoo.
See? A non-issue. The site will live without Google or Yahoo, or it will die with them. There really isn't any middle ground worth considering.