Is there a way to disable the end-user pages temporarily but keep the administration pages active? 

Environment:

For February 2012 release and newer

Resolution:

When promoting changes to your production site, you can manually turn the splash page on and off to reserve time for any post-promotion tasks. A configuration setting called "CP_MAINTENANCE_MODE_ENABLED" can be utilized.

Once enabled, all page requests within Production mode in Customer Portal will return the content of the splash.html page and all syndicated widgets will stop displaying data. This will not affect WebDAV, the CP admin area, custom controller endpoints, or requests within development or staging mode. Consequently, administrators can test or change Customer Portal before allowing it to be viewed by the end-users.

Path to setting(s): Select Configuration from the navigation area > Site Configuration > Configuration Settings > and search by Key.

For more information on enabling maintenance mode and customizing the splash page, refer to Answer ID 1306: Using a custom splash page for site maintenance and updates.