If you’ve sold or given away a computer, or if it’s died, you may want to deauthorize it to remove it from your iTunes Store account. You can’t do this for a specific computer, but you can deauthorize all your computers.
In iTunes, choose Account > View My Account. Enter your Apple ID and password, and you’ll see an Account Information page. Look on that page for the Computer Authorizations section.
This tells you have many computers have been authorized to use your iTunes Store account. However, it doesn’t tell you which ones.
Click Deauthorize All to deauthorize all your computers, whether you’re currently using them, or you’ve sold them, given them away, or retired them.
You’ll need to reauthorize the computers you’re still using; just start playing any content from the iTunes Store to display a dialog asking you to do this.