I am not sure if this is really what you are looking for: You can also make
a horizontal line from margin to margin by hitting the "minus" key at least
three times and then pressing Enter. Make sure that the Border Lines box in
the AutoFormat As You Type tab is checked. The 6 characters below can be
used to make horizontal lines. To do so just type the character at least 3
times and them press Enter. If you just want the dashes or characters press
Undelete. You can get different lines by using these characters in this
way: -, _, =, #, *, and ~
Note that this procedure inserts a Border Line; you can't delete it as you
would usually delete any ordinary characters. To delete horizontal lines
position the cursor above the unwanted line. Click on Format, Borders and
shading. Click None and OK back to the document. The line should
disappear.
For a more detailed explanation see
http://word.mvps.org/FAQs/Formatting/CantGetRidOfLine.htm