How do I deauthorize all computers on iTunes?
From this Apple Support article you can do it from 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.


The process works as follows


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




Also note the following:


  You may only use this feature once per year. The Deauthorize All button will not appear if you have fewer than 5 authorized computers, or if you have used this option within the last 12 months. If you need assistance on using this feature please contact iTunes Store support via email.