Can I remotely deauthorize computers that i no longer own?

Not necessary.

De-authorizing Computers (contributed by user John Galt)

You can de-authorize individual computers, but only by using those computers. The only 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, 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.

About deauthorizing specific computers - https://discussions.apple.com/thread/3809988 - Only, if you have physical access to the computer or are running VNC, otherwise you have to deauthorize all computers. De-authorize all computers button may not appear until you actually have 5 authorized and you can only do this once a year.

iTunes Store: Authorize or deauthorize your Mac or PC - http://support.apple.com/kb/HT1420 - and another helpful post: https://discussions.apple.com/thread/3798085?answerId=17828050022#17828050022