I don't know a lot about spiders, but I would think if your hidden forums aren't available to guests then they are hidden from spiders.
As far as ads, the only thing I can think of is to find the template(s) with the ad code and enclose them in a conditional that checks the forum ids, like
Code:
<vb:if condition="!in_array($foruminfo['forumid'], 1, 2, 3)>
// code hidden from forums 1, 2, and 3
</vb:if>