Have you tried:
Create a new group "readers", with "read" userright.
Disable "read" rights for "users" group.
Put your open content in the $wgWhitelistRead variable (login, main pages,
etc); make sure anything sensisitve isn't in $wgWhitelistRead.
To "activate" a new account, a burecrat has to add them to group
"readers".
To "deactivate" a new account, a burecrat removes them from group
"readers".
See:
http://meta.wikimedia.org/wiki/Help:User_rights
LocalSettings.php
includes/DefaultSettings.php
I haven't tested this quite yet, but was going to this week for an internal
corporate MediaWiki I'm running.
If it doesn't work, let me know, so I can figure something else out. 8-)
--
-george william herbert
george.herbert(a)gmail.com