Automatic Logoff in a Group Policy

Hey everyone,  I have searched the web and have not found a solution that works to my particular problem.  Here is the issue, I apologize as it may seem a bit confusing.  We have some public workstations in our office that anyone can logon to using their network username/password.  We would like to have the workstation automatically log them off after 2 minutes of inactivity.  Here's the catch.  We already have a group policy set on all users to have a screen saver run that locks their computer after 10 minutes of inactivity.  
So far I have tried putting the public WS in an OU and setting a GP to kick off the winexit.scr after 2 minutes.  However the GP that is applied to the users seems to override this and just kicks on the lock screen saver after 10 mintutes.  I tried taking myself out of the OU for the users, but the GP for the public WS still does not kick off the winexit.scr  I also set the public WS GP to not be overridden, but that doesn't work either.  It seems as though a screen saver GP will only work when applied to users and not computers.  Is this the case?  If so can anyone think of a workaround?  If worse comes to worse I wouldn't mind running a logon script that uses one screensaver when they are at their private WS and one when they are at any other WS.  I would just need some help writing it. Thanks in advance.