Interesting that this never existed as an option before. You had to actually remove the file and remove the entry from your eFiction install in order to truly disable new user registration.

This setting is in action on this very archive, because no one should try to register to a demo archive... but I will create a manual writeup to enable it on your own archive after this is pushed to the efiction3-mod repository.

For the record, this setting is located in Admin > Settings > Site Settings under Enable New User Registrations. Quite straightforward.