The Artisteer designer basically duplicates the horizontal menu in the vertical menu which to me doesn't make much sense, it seems more natural that the vertical menu be a sub menu and only show child pages rather than having 2 menus that do the same thing.
However if you want the vertical menu to not be a sub menu you can edit the layout.master file and remove this property from PageMenu control : IsSubMenu="true"
Also if you want the horizontal menu to also show child pages you can remove this property from the SiteMenu control: DynamicDisplayLevels="0"
Hope it helps,
Joe