how can i deauthorize all computer from my account

Deauthorize all computers

To deauthorize all your computers, follow these steps:

1. Click to open your account in iTunes (you may need to sign in with your Apple ID), or follow these steps:
   1. Open iTunes and click iTunes Store.
   2. Click Sign In and enter your Apple ID.
   3. Click your Apple ID and select Account.
2. Click Deauthorize All.
   The Deauthorize All button appears only if you have more than two authorized computers.

NOTE: Apple limits this option to once every 12 months.

This may help as well... http://support.apple.com/kb/HT1420