How do I deauthorize a computer from iTunes when I no longer have that computer?

Unfortunately, you can't without deauthorizing all your computers then re-authorizing the ones your still own.

Deauthorization

You can de-authorize individual computers, but only by using those computers. Your other option is to "de-authorize all" from your iTunes account.

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 "De-authorize All".
5. Authorize each computer you still have, as you may require.

You may only do this once per year.

After you "de-authorize all" your authorized computers, you may re-authorize each one as required.

If you have de-authorized all computers and need to do it again, but your year has not elapsed, then contact: Apple - Support - iTunes - Contact Us.

For more information on authorization and de-authorization: iTunes Store- Authorize or deauthorize your Mac or PC.

[Permission to use part or all of the above has been granted by Kappy, exclusively, to theratter.]