There are three options for this:
Create a custom home page. This is not ideal because the categories and
products would need to be added to the home page manually rather than
by checking the box to assign them to the root category.
are skins that are available that allow you to customize the height of
the banner. The simplest approach might be to choose one of those and
have a banner with a height of just a couple of pixels. The banner could be
transparent or colored to show a line. If you are looking at the skins
through Sites > sitename > Skins, the Customize Skin window has a
View Notes button at the upper-right that shows any detail for that skin
and how to adjust it.
3) Use a custom skin.