Do arquivo de ajuda do OOO (Writer):
To Stop Drawing a Line When You Type Three Identical Characters
OpenOffice.org automatically draws a line when you type three of the following characters and press Enter:
- _ = * ~ #
- Choose Tools > AutoCorrect Options.
- Click the Options tab.
- Unmark the “Apply border” check box."
Confira "linhas / bordas automáticas no texto" para saber mais.