How to remove known networks from Mac?
Just unlock (bottom left) the network prefs before hitting the advanced button.
You have to enter a login name with admin rights and the respective password to do so. If it's your personal Mac usually the one and only existing account has admin rights (though the login name doesn't contain 'Admin'). An office/school/university Mac has - depending on the internal guidelines - often several accounts: non-privileged standard user accounts and one or several admin accounts. Only the admin accounts are eligible to change network preferences.
Choose your WIFI-interface and click the advanced button
Remove any unwanted networks