you cant delete cookies on another users computer, the would need to clear the cache of the browser in oder to delete the cookies.
when you run the security assist wizard, go to the manage pages tab, and edit the login page, this screen will allow you to unselect the option for Remember me and auto login.
for pages that are already created, you can remove the auto login option by deleting the second Security Assist Authenticate user behavior on the page and deleting the auto login check box.