As far as I know you can't prevent IE from using its cache in all situations and you can't "turn off" the back button. Thus, if you really need this functionality you'll have to do it the hard way: put a serial number in each form and keep track of which form numbers have been submitted in the user's session. If the user tries to submit the same number twice then you know they went back. I think this is how Citibank's online banking system works, but I haven't looked too close.

-sam