Give the users a button that executes the "Exit Application" script step. This is the most direct method.
Other methods include training users to log out by clicking their account name in the menu bar and choosing "Logout" or by setting a sufficiently short timeout on the server that it has a minimal impact if they don't.
Also, you can set a homeurl parameter when the user logs in to webdirect so that when you call exit application, you get a more graceful page than the standard webdirect homepage.
I covered it a bit in my blog post here: