to add to this, I can't seem to find a way to deauthorize a computer remotely
There's an option in the iTunes Music Store Account settings to deauthorise all the computers you have authorised. I had to do this recently - http://www.apple.com/itunes/features/ says
Authorize and Deauthorize : Purchases you make from the iTunes Store can be played on up to five computers that you authorize using your iTunes Store account name and password. To authorize or deauthorize a single computer, choose Authorize Computer or Deauthorize Computer from the Store menu. If all 5 authorizations are being used, you can deauthorize all your computers at one time by choosing View My Account from the Store menu and selecting Deauthorize All.
Doesn't stop your problem when you get past five machines, but really who is actively using that many machines to manage their music?