Taken from - http://blogger-hints-and-tips.blogspot.in/2012/07/close-the-gap-left-when-Bloggers-navBar-is-removed.html
You can move the header up to cover over the area where the nav-bar was by adding a CSS rule to your template in the normal way.
The rule to add is:
#navbar { height: 0px; visibility: hidden; display: none;}
Apply that, and the space that the nav-bar was in will simply close up, so the rest of the content is a little closer to the top of the screen.
