Expire data from form

Hi, I have set up a form to register the users to the site. The users are allowed to register only once. When the user submits the form the same script will check the validity of the form and displays error messages and if okay, updates the database. I want to set up an option in the form in which when the user once submits and after that if they click on the back button (browser back) it should say the 'page expired' as I have seen with some Perl scripts. The users should not be allowed to go back to the page and resubmit. Can any body suggest a way to accomplish this. thanks Sheni R Meledath sheni@cyber-gear.com