The answer is 'not easily' - this is a function of vbulletin itself. I could modify the header template to exclude forums I think, or there was a mod available previously to allow it, I don't know if it's been updated to vb4 but will take a look when I get a chance. I'm not a huge fan of mods as they tend to make upgrades more difficult...

However, if you go to the front page - http://www.free-dc.org which is vbAdvanced, that does indeed have this feature and the main widget which shows new posts does not include those forums.