Did you consider using the TMOUT environment setting? The default shell from HP-UX 10.0 is sh-posix
(OSF POSIX), and TMOUT can be used to disconnect idle login sessions. This should also work in earlier versions of HP-UX that make use of ksh
You would simply need to set this environment variable system-wide (in /etc/profile) or in the user's individual profile file to cause an idle login session to be terminated after a specified amount of seconds:
# Set TMOUT to 15 minutes, read-only so user cannot override it.