In both 2003 nad 2007, you can right-click on the section (i.e. tab across
the top of the screen) and select "Delete" from the context menu. The
corresponding *.ONE file will be moved to Window's Recycle Bin.
In case wanted to ask on how to delete a section in OneNote:
In ON 2007:
Just right-click on the section tab and select "delete" from the
context menu.