There's no doubt that Apple's iTunes Music Store is a success. With billions of tracks sold, and millions of iPods around the world, Apple's position as the number one online music retailer doesn't seem to be going anywhere soon.
However, not being able to authorize your iTunes store account on more than five computers that might need access to your protected music tracks, movies, and applications won't be going anywhere either. Fortunately, Apple has a pretty easy solution for deauthorizing machines to which you still have access, but in case you can't get to a far away machine, Apple has created a kill switch to deauthorize all of your machines at once. We'll show you step by step instructions for both the easy way, and the hard way, to deauthorize all your iTunes accounts at once.
First, lets talk how the iTunes Music Store works. You sign in with an Apple ID, which links your credit card details, among other information, to an email address that lets you purchase music, movies and applications from the iTunes store. Apple allows this account to be linked to five separate machines. To link the machines, it's a relatively simple process. 
Either attempt to play a song that's authorized by another user, which will then throw up a prompt asking you for an Apple ID and password, or head to the "Store" menu and select "Authorize Computer." Deauthorizing individual machines is also a simple process, as long as you still have access to each machine. Again, head to the "Store" menu, and select "Deauthorize Computer."
Every user on a computer will have to be authorized to play music, as it'll be linked to each individual user account and not linked to the computer at large. Also, if you delete that user account without deauthorizing iTunes, the authorization will dissapear, and will be unable to be restored without following our guide. Also, keep in mind Apple says you can only deauthorize all machines if you've hit the five machine limit, and that you can only use the "kill switch" once every 12 months.
I ran into authorization problems after attempting to sync an iPod to my new MacBook Pro. iPods using auto sync will automatically delete any non-authorized material from the iPod, and if the iPod syncing with an unknown library, all the content will be deleted after being presented with a warning message. To avoid losing data in either scenario, simply disable autosync and manage your library manually. I, however, like the ability to automatically sync my iPod whenever its plugged in, so I attempted to authorize the machine, which is when I found out I had hit my activation limit of five machines.
The first step to remedy your problem is to head to the iTunes Store section of iTunes, click the "sign in" button in the upper right corner, and log in to your account.
After you've signed in, click on your username, again in the same spot in the upper right corner, and you'll see your account settings. You'll also see the number of authorizations you've completed, and a big button that says "deauthorize all." If you're sure you're ready, go ahead and click the button.
iTunes will ask you one more time if you'd like to deauthorize all accounts. This will disable any machine connected to the internet that you have previously authorized. Again, you can only use this "nuclear option" once every 12 months, so be sure this is the time to use it before confirming the dialogue box.
Just like that, you're set. You'll see a confirmation message, letting you know all your computers are deauthorized. Now, all that's left is to authorize your new machine and get back to syncing your music.
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