Simple answer: You can't, or at least not against the determined.
HTTP is a stateless protocol, and the webserver, and therefore your CGI are basically unable to tell the difference between one user's browser and another
I believe the closest you could come is to use cookies, and to force users to accept cookies in order to view the form.
This is defeatable, because the user can freely edit their own cookie jar, and forcing cookies could also offend some users.
davis