iTunes Store deauthorization

http://support.apple.com/kb/HT1420

To deauthorize all computers associated with your account

If you find you have reached 5 authorizations, you can reset your authorization count by clicking Deauthorize All in the Account Information screen.

   1. Click iTunes Store in the menu on the left side of iTunes.
   2. If you're not signed in to the store, click the Account button, then enter your account name and password.
   3. Click the Account button again (your ID appears on the button), enter your password, and then click View Account.
   4. In the Account Information window, click Deauthorize All.