[ozzy47]

You would have to add two new plugins. ACP --> Plugins & Products --> Add New Plugin

This is untested, but it should work.

Product: vBulletin
Hook Location: postdata_presave (excludes replies to existing threads)
Title: Exclude Forum Posts From Activity Stream
Execution Order: 5
Plugin PHP Code:

if (in_array($this->info['forum']['forumid'], array(X,Y,Z)))
{  
           $this->set_info('skip_activitystream', true);  
}

Product: vBulletin
Hook Location: threadfpdata_presave (excludes new threads)
Title: Exclude Forum Threads From Activity Stream
Execution Order: 5
Plugin PHP Code:

if (in_array($this->info['forum']['forumid'], array(X,Y,Z)))
{  
           $this->set_info('skip_activitystream', true);  
}

X,Y,Z are of course the forums you want to exclude. This will only work on new threads/posts not existing ones.