I'm trying to lock the header and footer on a Word document so it can't be accessed or changed

The restrict editing feature does exactly what you want.

Make the body of the document an "Exception." Select the entire body of the document (Ctrl+A) . Mark that as an exception for everyone.

This is how it's done!

1. Open the Word document that contains the header and the footer that you want to protect from changes. Do not open the header or the footer of the document.
2. Make sure that you are at the top of your Word document. To do this, press CTRL+HOME.
3. On the Page Layout ribbon, click Breaks.
4. In the Breaks menu, click Continuous under Section breaks, and then click OK.
5. A continuous section break is inserted at the top of your document.
6. On the Review ribbon, click Restrict Editing.
7. In the Restrict Editing task pane, click to select the Allow only this type of editing in the document check box, and then change the box to Filling in forms.
8. Click Select sections.
9. In the Selection Protection dialog box, click to clear the Section 2 check box, and then click OK.
10. Under Start enforcement, click Yes, Start Enforcing Protection.
11. Assign a password and OK

This is how it's done!

1. Open the Word document that contains the header and the footer that you want to protect from changes. Do not open the header or the footer of the document.
2. Make sure that you are at the top of your Word document. To do this, press CTRL+HOME.
3. On the Page Layout ribbon, click Breaks.
4. In the Breaks menu, click Continuous under Section breaks, and then click OK.
5. A continuous section break is inserted at the top of your document.
6. On the Review ribbon, click Restrict Editing.
7. In the Restrict Editing task pane, click to select the Allow only this type of editing in the document check box, and then change the box to Filling in forms.
8. Click Select sections.
9. In the Selection Protection dialog box, click to clear the Section 2 check box, and then click OK.
10. Under Start enforcement, click Yes, Start Enforcing Protection.
11. Assign a password and OK

You didn't say that you wanted to keep them from clicking in the header/footer. The macro at http://word.mvps.org/FAQs/Customization/ProtectWord2000PlusHeader.htm will bounce someone out of a header/footer.

Your method also limits editing in the main document and makes the spell checker not run unless you add special macros. Tho other features that are locked by protecting for forms cannot be recovered while the document is protected.

BTW, if you sent me a copy of your protected document locked either way, I believe I could access/edit the header/footer in less than 3 minutes. A basic rule is that if someone can see something, they can copy it.