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:
Go to Settings
Head to the Collaborate section
Hover over the user until the green pencil icon appears and click on it.
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"
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 until you see a red "X" logo appear. Please click this red logo to remove the user from your team completely.