Press the Windows Key and type "CMD.EXE" and "Run as administrator". Enter your administrator user credentials when a prompt appears and/or click yes.
On the CMD window enter lusrmr
On the lusrmgr window, select "Users" folder and right click the user you wish to remove admin rights from, and click properties.
On the Properties pane, select “Member Of”, click on “Administrators” and click “Remove” button.
