which also gets ignored by rogue bots..
ideally, you simply make all your forums require a user sign up.. you might also enable captcha at both signup and posting for a default 'lower than whale dung' group
no guests at all allowed..
then at most a rogue bot will 'crawl' the open stuff, which ain't much.. maybe you have a portal page with articles about basket weaving and such
i assume you site has a lot to do with government secrets?