Your Mac automatically reconnects to Wi-Fi networks you’ve previously connected to. Starting with macOS High Sierra, you can now tell your Mac not to automatically connect to certain Wi-FI networks. Your Mac will remember the Wi-Fi network’s passphrase and other connection details, but won’t connect unless you tell it to.
This is useful for Wi-Fi networks you may use occasionally, like Comcast’s xfinitywifi or other public Wi-Fi networks. On older versions of macOS, you had to delete a saved Wi-Fi network and its passphrase to stop the automatic connection from occurring.
First, open the System Preferences window by clicking Apple menu > System Preferences.
Click the “Network” icon in the System Preferences window.
Select the “Wi-Fi” option in the left pane and choose the Wi-Fi network you want to modify from the Network Name box.
Uncheck “Automatically join this network” and your Mac won’t automatically join the Wi-Fi network in the future.
You can also set the order of your Wi-Fi networks to tell your Mac to prefer certain Wi-Fi networks over other ones. Your Mac will try to connect to available Wi-Fi networks in the order you choose, preferring the ones at the top of the list.