This plugin will hide the following things from guests when they will browse your forum:
1) External Links.
2) Images. (All images posted by the way of the img tags and the attachments as well)
3) All content posted by the way of the Code tags.
Guests will see a login or registered message instead. The above mentioned things will be hidden in the search results too from guests.
You can exclude a certain forum or forums from hiding those things from your guests.
To install this plugin, log in to your Admin Panel, go to System->Plugins, click the Install New Plugin button and import the plugin 's xml file.
To configure it, click the Edit button of the newly installed plugin and you will be taken to the plugin 's Settings.