How do I deauthorize all computers on my account?

http://support.apple.com/kb/ht4627

Removing an associated device or computer from an Apple ID

To remove a device or computer from your Apple ID:

1. Open iTunes.
2. Sign in to your Apple ID by choosing Store > Sign In from the iTunes menu.
3. Choose Store > View My Account from the iTunes menu.
4. From the Account Information screen, click Manage Devices.
5. Click the Remove button next to the device name you would like to unassociate.