I once made a similar post. That trick no longer works since blogger.com’s upgrade. Now I have a new cleaner and better trick to make the top banner disappear. Add a new HTML/Javascript widget to your template. Now in this template add the following code.

<!--exclusively BY APPLEGREW (applegrew.blogspot.com),
HIDES Navbar -->
<style type="text/css">
div#navbar { display: none; }
</style>
<!--end of HIDE Navbar-->

Now save the widget and reload the page.

A tip. The navbar won’t disappear until this code gets loaded, hence make it gets loaded faster. Simply put it in head portion of your template.