How do i force a guest account to Auto log off after being inactive for a period of time?

I have a 2010 21" iMac running OS X Snow Leopard (10.6.8) in a hotel's guest lounge. I want to have it log off of the guest account after being inactive for 5 mins so that it clears the session info if the last person who used it did not log off when they were done (***this happens very often***).

I tried using the "System Preferences > Security> Log off when inactive for X minutes" option but this brings up the "Delete files and log off" dialogue which unless someone makes a selection just stays there and doesn't timeout, essentially stopping the logoff process.

How can i bypass that dialogue and force the system to log off the Guest User to avoid any issues with other guest seeing info from the previous user?