How to remove network user as Administrator

In the process of trying to solve a different question, on respondant suggested I change the 'normal' user to an Admin user. That idea didn't work, but now I cannot change the user back to a non-admin user. When logged in as that user: System Preferences > Users & Groups, the "Allow user to administer this computer" is checked, but grayed-out. If logging in as actual Administrator this other users does not show up at all in System Preferences > Users & Groups, probably because he is a network user.

How to I now set this user as a non-Administrator?