Normally you have to deauthorize on the actual computer, but can you deauthorize one conputer from another? I caanot get into the one I want to deauthorize and hope to be able to remove it from my account.
Once a year though, you can deauthorize all of your machines in your iTunes account management. If you haven't hit your 5 computer limit yet I wouldn't worry about it.
I did it through a phone call to an apple sore. The lady used my account and password for the extra account and deleted it. Now I have only one account and one computer.