If you use the FP server ext form feature to save the form results to a text
file in the _private folder, no one should be able to see it, because the
permissions on the folder are set as such that no one can view the contents
of that folder through a web browser (most likely a "Forbidden 403 server
error" will result if any attempt is made).
You can access the file through Frontpage itself. and publish the file to
your local hard drive.
If you're using a database (eg, MS Access) and the Database Results Wizard,
you can pick and choose the fields you want displayed and those you don't.
If the wizard places initially places everything on the screen in a results
area, you can just delete the private field you don't want shown (this
doesn't deleted it from the database, it merely doesn't show it on the
screen.). However you have control in the DBRW to display on the fields
you want in the initial screens of the DBRW.
If you want it more secure, assign a password to the database file itself
which means you will be prompted fo the password each time you open the
Access database offline. No one should get into it with ease, if the manage
to get hold of the db file itself.