I usually see those appear when I enter three asterisks and hit return for a scene break. When it does appear, I hit Control-Z (or Edit|Undo), and it goes back to the asterisks.
Ah. Here's some more information, from Office Help for Office 2003:
borders
Applies a border above a paragraph as you type three or more of the following characters:
•Hyphens or underscores for a single line
•Equal signs for a double line
•Asterisks for a dotted line
•Tildes (~) for a single wavy line
•Number signs (#) for a decorative line
Note Automatic borders can only be applied as you type (AutoFormat As You Type tab).
I'd suggest checking Format|Borders & Shading. I know you're in 2007, but if you hit the key sequence Alt-O-B without the hyphens, that should still get you into the right place.