How to Deauthorize Computers on iTunes That You Don't Have
You can authorize up to five computers to play content purchased through your iTunes account. If you authorized a computer you no longer have access to, you must clear all of your computer authorizations, then reauthorize any computers that you still want to use to play content. You can only deauthorize all of your computers once per year.
Open iTunes. Click "iTunes Store" on the left-hand side of the iTunes program window.
Click "Sign In" in the upper-right corner of the program window, if you are not already signed in to your iTunes account. Enter your Apple ID and password in the dialog box that appears on the screen and click "Sign In."
Click the name of your account in the upper-right corner of the screen and enter the password for your Apple account if prompted. Click "View Account."
Click "Deauthorize All," which is located to the right of "Computer Authorizations." Click "Deauthorize all computers" in the dialog box that appears. Click "OK" when a dialog box appears saying that all computers have been deauthorized.
Open iTunes. Click "Store" in the menu bar in the upper-left corner of the screen.
Click "Authorize This Computer." Enter your Apple ID and password if prompted, then click "OK."
Repeat the process to authorize any additional computers to play iTunes content.