Word 2007 uses Content Controls to display document fields inside the document (eg. the header). These document fields can be standard fields but also your SharePoint Fields.
By default when the value of a Content Control is empty it will display the Placeholder Text between square brackets as follows in a sample document header:
It is possible to change this Placeholder Text by going into Design Mode and replacing the text in the Content Control.
Make sure that you see the Developer tab in the Word Ribbon. If you don’t see it you can turn it on in the Word Options;
Open the Developer tab and enable Design Mode;
Now the text displayed in the Content Control is actually that one for the Placeholder Text and can be modified as desired. Once you leave design mode the Content Placeholder will return to displaying the original value and have saved the new Placeholder Text.
Removing all text from the Content Control Placeholder Text will result in an error when leaving Design Mode;
The closest you can get to making it appear empty is by using a single space character. Once you exit Design Mode you’ll see the result for those fields that have no value set.