Title: Limited Login
Subject: I want to block users form logging in while I'm backing up a file.  I
don't want to put the system in single user mode.
 
There is a way to get the same effect by adding a few lines to the
/etc/profile and /etc/csh.login scripts.
for the /etc/profile:
name=`logname`
if [ $name != root ]
then
  echo "$name not allowed to login, System is Unavailable"
  echo "Exiting"
  sleep 2
  exit
fi

for the /etc/csh.login:
set name = `logname`
if ( $name != 'root' ) then
   echo "$name not allowed to login, System is Unavailable"
   echo "Exiting"
   sleep 2
   logout
endif

This is not a "supported" method by the Response Center, but I have
tried it on my system and it does work.
Please let me know if this answers your question.