https://imore.com/how-stop-auto-joining-wifi-network-mac is such a helpful article! One more question though: Can you uncheck "Automatically join this network" for a network like "XFINITY" which requires you to immediately enter credentials, otherwise the OS bounces you off of it, so you can't uncheck the box. I get: "The Wi-Fi network 'XFINITY' requires WPA2 enterprise credentials." And then it asks for Username and Password, without which, High Sierra doesn't allow you to uncheck "Automatically join..."