You can do this using the Restrict Editing button on the Developer tab. The Developer tab isn't visible by default--if you don't see it, the option to display it is under File | Options | Customize Ribbon (Microsoft Support article with steps).
- Format your headers and footers the way you want them.
- Select Restrict Editing on the Developer tab.
- Under 2. Editing restrictions, select No changes (Read Only).
- Select the body of the document. Under Exceptions, check Everyone. (If you have formatting marks displayed, you'll see a gray I-bar appear in the document.)
- Under 3. Start enforcement, select Yes, Start enforcing protection. Enter a password if you want one, or leave it blank for no password. (Not selecting a password means that employees can remove the protections if they're that bound and determined to mess with your formatting, and if they know how.)