Hi:
the following command allows a user to login only once:
# all on one line
[ `w -h -s| cut -d" " -f1 | grep "^$LOGNAME$" | wc -l` -gt 1 ] && exit 0
You should be able to modify it to fit your needs. Since the "w" prints a summary of who is logged into a system, if more than one instance of the user exists, terminate the user's login process.
If it were me, I'd place this in /etc/profile. If you place it in the user's .profile, they can edit it, if said user has command-line access.
Regards,
Ed