One of my account holders had her old account locked and deleted several months back. She registered a new account, but was unable to enter her preferred email address since it is apparently still listed in the database as being already in use. I pulled up the "deleted" account and was able to change the address associated with that account so that it is no longer associated with her preferred email address. A member search through the admin panel does not show her email address associated with any accounts, currently. I am still unable to update her new account with her preferred email address, as it is still listed as "already in use". Is there a way for me to manually remove it from the database?
You'd need to access the database, and it seems you don't know how to do that. I suggest you temporarily remove the unique email restriction (Admin Options -> Email Server configuration), set the address you want and then restore the restriction again.
I can follow instructions just fine, and would have appreciated an answer to the question I asked in case this problem should arise again. However, this is a quick and simple solution which accomplished the same means so... thanks for that. Problem solved.