You can only Deauthorize computers you have in your possession.
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) and then re-authorize your current Computer(s) one at a time.
Authorise / Deauthorise About
... . Is there a way in which I can find which device is linked to my applee account?