For computers: You can de-authorize individual computers, but only by using those computers. Otherwise you can "deauthorize all" from your iTunes account.
- Open iTunes on a computer
- 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.
It is also possible to have mistakenly authorized a computer more than once. To fix that:
- Open iTunes.
- From the Store menu, choose "Deauthorize This Computer…"
- Enter your account name and password.
- Click OK. A message appears reading "This computer has been successfully deauthorized."
- Click OK.
- 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."
For iOS devices: You can authorize and deauthorize individual devices under iTunes in the Cloud. You will find a link on your iTunes Account page under Manage Devices.
iTunes Store: About authorization and deauthorization
One computer using multiple iTunes Store authorizations