As they are an Admin of the account, you won't be able to delete them until their permission is changed. To change the permission, please follow the steps below:

  4. Under the timezone and password for the user, you'll see an option to give                 them "Admin access" or to give "User level access"
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    5. Please select "User level access"

    6. Save the changes and go back to the main Collaborate page. 

    7. Now hover over the user again and then click delete.