And another thing to do before you continue on is turn off Border Lines in your AutoCorrect options:

Word 2007 - Office Button > Word Options > Proofing > AutoCorrect Options button > AutoFormat As You Type > uncheck the Border lines box

Word 2003 - Tools > AutoCorrect Options > AutoFormat As You Type > uncheck the Border lines box

These are the lines that get made when you make several lies (minus sign, equals sign, underscore) in a row, then press enter and you end up with a continuous line across the page.

If you find that zyzzyva57's method works, but every time you add a line break at those points the line comes back - then copy and paste the entire document into Notepad, save it, close it, open it (still in Notepad) and then paste it back into a new Word document - this will remove all formatting AND formatting behavioural tags. (just don't forget to turn off Border lines to prevent it happening again! LOL!)