You can de-authorize individual computers, but only by using those computers. Otherwise you can "deauthorize all" from your iTunes account.
- Open iTunes
- From the Store menu, select "View my Account..."
- Sign in with your Apple ID and password.
- Under "Computer Authorizations" select "Deauthorize All".
- Authorize each computer you still have, as you may require.
You may only do this once per year.
After you "deauthorize all" your authorized computers, authorize each one as required.