Prevent changing of answers

How do you prevent someone from changing their answers for a fill-in once he/she has answered and submitted their results in a test? I have created an action that returns users to a "Good bye" page if they hit any key and the test has been processed and submitted. This works 90% of the time, but special characters like =][ and the delete key still allow that item to be entered or a delete to occur (if the delete key is used). Any thoughts?

Thanks,