I can see how to do it if you did NOT have randomized pool. Here is the solution w/o randomized pool. Also, I am not totally sure that if you exit test and return that the same questions will be drawn from the pool. Best test that out.
Now to the solution. Create a grayed-out Submit button and place it over the real Submit button. Put this on a page after the last question. Put an On Show action on the page that Hides the grayed out button with a condition that consists of each question variable being Not Empty. This will not work when you are selecting some subset of questions from a pool because you will not know which question variables to put in the condition.
Any one else have some ideas here????
