How to Restrict unregistered users from accessing any of the Joomla website?
Good grief, so much offered when so little is asked.
No, taking the site offline restricts access to all but Super Users.
Who cares why he wants to do it. He asked if it can be done.
Here's a simple way to restrict access. It is written for J2.5 but the idea remains the same. Set permissions to be restrictive.