About deauthorizing specific computers - https://discussions.apple.com/thread/3809988 - Only, if you have physical access to the computer, otherwise you have to deauthorize all computers. De-authorize all computers button does not appear until you actually have 5 authorized and you can only do this once a year.
iTunes Store: Associating a device or computer to your Apple ID - http://support.apple.com/kb/HT4627 - "Your Apple ID can have up to 10 devices and computers (combined) associated with it. Each computer must also be authorized using the same Apple ID. Once a device or computer is associated with your Apple ID, you cannot associate that device or computer with another Apple ID for 90 days." - Additionally instructions for "Removing an associated device or computer from an Apple ID"