This is not a problem! Simply go to the iTunes Store and access your Apple Account Information screen. If you've authorized five Macs, a Deauthorize All option will appear - click it to deauthorize all of them. You'll need to reauthorize all your other computers, however, and you can only do this once a year.
BONUS TIP:
Enter Vanilla Ice in the Search field. Press Return, then Command-A. Choose Edit > Clear, and click Remove in the dialog that appears. Believe us, you'll feel better.
