... make it simple for those who accidently don't deauthorize before selling or letting their computer go (or the computer breaking before it can be deauthorized)...
If you no longer have, or access to, 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)
Then re-authorize your current Computer(s) one at a time..
Authorise / Deauthorise About