From my understanding, the navigation pane, for a lack of better terms, acts like a mini TOC that is basically compiled throught the use of STYLES. So, in order to remove the navigation pane headings you would have to eliminate the styles OR, redefine the styles so that they would not appear in the TOC/Navigation Pane. Based on your comment ref the size of your document, this would not be advantageous or, would be very cumbersom/tedious.

Just my thoughts...