You can de-authorize individual computers, but only by using those computers. It is best to wait until you have authorized the maximum of five computers before resorting to using "deauthorize all" from your iTunes account, since you may only do that once in twelve calendar months.
1. Open iTunes on a computer
2. From the Store menu, select "View my Account..."
3. Sign in with your Apple ID and password.
4. Under "Computer Authorizations" select "Deauthorize All".
5. Authorize each computer you still have, as you may require.
After you "deauthorize all" your authorized computers, authorize each one as required.
It is also possible to have mistakenly authorized a computer more than once. To fix that:
1. Open iTunes.
2. From the Store menu, choose "Deauthorize This Computer…"
3. Enter your account name and password.
4. Click OK. A message appears reading "This computer has been successfully deauthorized."
5. Click OK.
6. Repeat steps 2 through 5 above. If you see the same message reading: "This computer has been successfully deauthorized," then your computer was using multiple authorizations.
Repeat steps 2 through 5 above until you see a message reading "This computer was not authorized. To authorize this computer, play a song or video you have purchased using this account."