Adsense irrelevant ads

[Cornolio]

I use to have irrelevant adsense ads on my forums.
I thought about customizing keys and descriptions for every forum.

like this?

PHP Code:

<if condition="$GLOBALS[forumid] == 3"><meta name="keywords" content="cinema, teatro, tv, televisione" />
<meta name="description" content="Forum su cinema teatro tv" />
<else />
<meta name="keywords" content="$vboptions[keywords]"/>
<meta name="description" content="$vboptions[description]"/>
</if> 


otherwise, how to disable forum access only for adsensebot (so it only crawls meta tags)? this should help, 'cause i heard that someone disabled forum access for unsubscribed users (so even for bots) to have relevant ads ('cause bots crawls only keys and description)