What is the best way to limit users to one session ?

You can also set limits on published apps (or published desktops) inside the properties of the app under "limits" tab.  Can limit each user to just 1 instance.
As for best way to log idle users off, best way is via the Properties of ICA-tcp under Terminal Services Configuration on each server.  Can also be done by GPO but is not a best practice.
As for encouraging users to log off, GPOs can be configured to add "log off" button to start menu and a few other options you'll notice under "start menu and taskbar".
You can also get a utility called "appscape" to remove the "X" in the upper right corner of the Citrix session so that users can't click the "X" to disconnect, but instead have to actually log off.