[xwiki-users] "Prevent unregistered users..." option not working in WikiPreferences > Rights

Hi, I have recently installed version 1.5 (on postgresql) and the "Prevent unregistered users from viewing pages,.." options are not selectable. I need to setup the Wiki so that it is private and does not accept new registrations. The checkbox fields are not clickable, I have also tried a fresh 1.6 install and I get the same behavior. Could someone please help me to lock down the Wiki? I have searched for this but all the answers and documentation seem to be out of date. Thank You.