How to show a Joomla module only for people who haven't logged in your website
Let's say that you want to make a module "Login" or "Join us" and you only want it to appear when the visitor has not logged in your site.
Or, maybe you want to display ads to those visitors.
Instructions For Joomla 3
How to hide a Joomla module for logged in users (registered)
In Users backend Joomla menu
1. Go to Users -> Groups -> New group -> Give the name "Guest". Make it a child of the Public group.
2. Go to Users -> Access levels -> New access level -> Give the name "Guest" and tick only the Guest group access to this level.
3. Go to Users -> User Manager -> Options -> Chnage the "Guest User Group" from the default value of "Public" to "Guest".
4. Go to the module that you want to make visible only for guests and select Access -> "Guest".
The same (step 4) can be done for menus, menu items and articles!