How can I hide the left column only for guests?

Follow these steps:

1. Navigate to the DefaultSkin page via shortcut and type "Skin" into the search field
2. Select Default XWiki Skin
3. Press "Edit this skin"
4. Add the new overriding template, name it layoutExtraVars.vm and place these lines of code in the template content. If the template already exists, you can place that code at the beginning or at the end of the existing content.
   #if($isGuest)
     #set ($showLeftPanels = '0')
   #end