Purely out of curiosity, but what would happen in all accounts were locked out from being able to be logged in to?? The root account, on most unix-based systems, is locked by default. What if all other login-enabled accounts were changed to be login-disabled?? If such a thing were possible, would there be a possibility of using some way to gain control over the system without being able to login?? Or would the system and its contents be inaccessible?? This is pure curiosity. Obviously, I would never do this if it were possible.
Most UNIX have root not locked unless using RBAC perhaps but accessible to only very limited devices where ordinary people have no access...
Thats is why servers are in white? rooms where security is high and only the sysadms have access...
To answer
The only system I found with root disabled was a lab machine configured with another sysadm of my team, we were trying all figures to go further in security... and so using RBAC we disabled root account... Fine till we forgot the passwords... And realised we were doomed, though my friend and collegue is highly specialized in solaris ( as I had no small HPUX to use at the time ) this was a sparc/solaris 10, we just could not find a way to get back at the system, OK we may did more than just RBAC as we were trying to highly secure... So on one hand we achieved what we wanted, on the other we saw the silly situation we were in for not remembering the passwords...
Morality?
I am sure all serious sysadm configures his boxes with a backdoor only it may not be at a user level, and the minimum security to my eyes is to not let direct connection from the net unless dedicated lan to root even via ssh, using su/sudo only and having root only accessible from console which should be a real dedicated console (/dev/console...) on serial port or dedicated lan for lan consoles