You can use the macro in
http://www.word.mvps.org/FAQs/Customization/ProtectWord2000PlusHeader.htm.
However, if the user has the macro security level set to High, the macro
will be disabled and the header won't be locked.

Are you worried about deliberate sabotage, or merely trying to prevent
accidental movement? For the latter, simply having the logo in the header
should be sufficient. There is no way to lock out a determined idiot.