Best way to disable going back to form page after user has clicked submit?

[eluser]Unknown[/eluser]
Hi guys, I've built a small survey form which will be sent out via email to a list of people. After the user has clicked submit on the "survey form" page, a "thank you" page is displayed. I'd like to make it so that if they click the browser back button the cached "survey form" page is not displayed. Maybe it will say the page has expired or something?

Sorry I'm fairly new to php/codeigniter! Any help would be really appreciated.

Thanks =)