i dont think there is a way of preventing authorized users from submitting the form, irrespect of wheter it is a valid form , or the form has been changed using notepad or any thing. I think u can use some checks like at time of saving the form , count the number of chars in the form, and at start time count again and check if the last count matches the new count. if not disable submit button or better dont allow him to view the form.
note: this is a basic approach, u can better it according to your skills and time availability