About deauthorizing specific computers - https://discussions.apple.com/thread/3809988 - Only, if you have physical access to the computer, otherwise you have to deauthorize all computers. De-authorize all computers button does not appear until you actually have 5 authorized and you can only do this once a year.
If you no longer have the computer(s) you want to deauthorise,
Log in to iTunes, go to "view your account info" on the itunes store, deauthorise all five, (Please Note: this can only be done Once every 12 months) and then re-authorize your current Computer(s) one at a time.
Authorise / Deauthorise About