Add a big label that says "Do not use your back button"

just a note: i hate websites that do that, especially if you get wrong results if you do use the button. i have a browser with back- and forward buttons (or keyboard shortcuts), and i want to use them. i might not be the only one.

i'm pretty sure every problem like this can be solved in a way so that the user might use their browser like it is meant to. otherwise HTTP is probably not the right protocol for the task.
just my 2 cents.