World Lock
This module is designed to allow GMs to lock a world and prevent non-GM users from joining the world (during campaign prep or for whatever other reason). When the setting is enabled in the world, it will cause any non-GM user that attempts to load the game to instantly log back out instead of being able to get into the world.
There is also the option for a visual indicator of if the world is locked, which defaults to showing a red border around the list of users.