I'm guessing you mean this:
Word 2007:
Office Button > Word Options > Advanced (tab) > Display (section) >
Set "Style area pane width in Draft and Outline views" to 0
Earlier versions:
Tools > Options > View (tab) > Outline and Normal options (section) >
Set "Style area width" to 0
More simply - click and drag the dividing line all the way to the (left)
edge of the screen.