In struts framework only one instance of ActionForm is created and same is shared among all user.
Whenever there is a request related to action form is recived by controller
servlet conrotter servlet calls the reset method of action form.
by default reset method does nothing we have to override it to get desired result (like reseting values to blank or their initial value).
I dont know this applies to portlet as i told you that I have no idea of portal.
It was just a gray thought that for a new user value that is set earlier being displayed.