You can't prevent someone from printing a file using the built-in Print
command (only if they're printing the file through a button you've created
in your file), but you can use a script that resets the form just before
the file is printed if some condition is not met.
Will this "reset" script simply highlight the unanswered fields? Is there an example of this script somewhere?
No, it will just clear the values of all the fields in the file so that the printed copy will be blank.
Regarding an example, see my PM.
"Will Print" actions run no matter what print action is used, even the "Ctrl + P" or File => Print.
You can then use the "Did Print" action to undo the action you used for the "Will Print".
You could hide some or all of the fields and then unhide the fields.