How to Deauthorize iTunes If I No Longer Have the Computer
During the transition to a new computer it is highly recommended to protect your purchased content in iTunes by deauthorizing your old computer, which removes the old computer's ability to play your files. As of 2010, Apple only allows you to share your purchased content with up to 4 other users per account if your content has digital rights management (DRM) protection.
Deauthorize Single User
Open iTunes. Click "Store" at the top of the screen, and click "Deauthorize Computer" from the drop-down menu.
Type in your Apple I.D., which is your email address, and your password.
Click the "Deauthorize" button.
Deauthorize All Users
Open iTunes. Click the "iTunes Store" tab located on the sidebar of the iTunes application.
Click the "Sign In" button, or the button that has your email address.
Enter your Apple I.D. and password, and click "View Account."
Click the "Deauthorize All" button in the "Apple Account Information" window.