If you are sure it is clip art, you should be able to select the object
(using the Select Objects tool on the Drawing toolbar if necessary) and
click Delete. It may be anchored to the header, in which case you'll have to
be in the header pane (View | Header and Footer) in order to delete it. If
you don't get a four-headed arrow when you hover over it, it's not a drawing
object or picture but more likely a border of some sort, in which case you
remove it by formatting the appropriate paragraph as No Border.