Joomla is pretty good at hiding things from the public, and then exposing them once the user logs in.  However, what happens when you want to hide a module position, for example, when the user logs in.  Andrew Eddie had this problem with his site" The Art of Joomla"  He posted a great solution where a few lines of code in the main template file is needed to hide it. We quote his solution:

To hide a module position all we have to do is work out if the user is logged in.  The Joomla Framework API (that's just a fancy name for some of the code that runs the Joomla web site) has a way of getting the user information.  All we need to do is check if the User ID is not zero and then we will know if they are logged in.  Let's see how this is done.

Open up the main template index.php file for your active template.  It will be something like:

/templates/ja_purity/index.php

You should already be familar for how Joomla displays a module position (with one of those <jdoc: ... /> tags).  Find the module position that you want to hide and then wrap it in the following code:

<?php
// Get the user object
$user = &JFactory::getUser();
// Now work out the User ID
$userId = $user->get('id');
// Now hide the module if user is logged in
if ($userId == 0) :
  // Now we break out of the PHP tags and display the JDOC tag to include the module
?>
<jdoc:include type="modules" name="public-right" style="xhtml" />
<?php endif; ?>

You will probably want to create a special module position.  In this case I've called it "public-right" to indicate it will display in the right column, but only for the public user.  When you log into your site, you should find that any modules in the "public-right" position will disappear.

This is a very simple technique but it can have a profound effect on the "feel" of your web site.