Yes. This is called Maintenance mode, one of the General Server settings on the Administration panel. It is often used during Forum Maintenance, or when installing or uninstalling forum modifications or themes.

• This setting is found in Administration Center > Server Settings > General
• Before enabling Maintenance mode, set the Subject for display and Message for display to explain to your users why the forum is unavailable.
• During the time maintenance mode is enabled, users will be able to login, but only administrators will be able to view, edit, or post the forum.
• Be sure to uncheck the Enable Maintenance Mode setting after maintenance has been completed.