How to exit vaadin Application from init() method cleanly?

This is for vaadin 6 Application.
I tried to add an authentication in init() method. When it fails, call the close() of Application class. But it seems that the close method called here does not really exit the application. If I run this Application again, it remembers the previous state and goes to the failure again.
So what is the proper method to close vaadin application cleanly without posting any panel?
Thanks for any help.