We removed a member from our Organisation as they are no longer part of our team but there are a few boards that this person was the only administrator. How can we remove the boards or add a new administrator to the board if it was not done before they left.
If you still maintain an e-mail account for this user you might temporarily re-enable it with a new password and then use that e-mail address with the "Forgot Password" login process with Trello. A change password e-mail should be sent. (You may need to know the username which should be in the original "Welcome to Trello" e-mail)
Once logged in to Trello as that member (you only removed the member from your organization, not from Trello) you can then add administrators of your choice to the existing boards you wish to transfer. You can also remove any boards no longer needed.
If you are not able to use the e-mail account or otherwise not able to log into the Trello account for that user, I suggest contacting firstname.lastname@example.org