automatic logoff

[Dick McGough]

I have 3 users on my computer; 1 admin account and 2
limited accounts. the guest account is turned off.

For security reasons, I need to have the limited accounts
automatically go to the login screen (and require
reentering a password) after a certain number of minutes
idle time.

When the administrator is logged on the system does
automatically logoff after x minutes; however, neither
limited user.

How do I accomplish this?
thanks in advance