OVERVIEW
You may need to remove a wireless network from your list of known/previously connected networks for either security or convenience sake. Instructions on how to do this are below.
INSTRUCTIONS
Click the Wi-Fi icon in your Mac’s menu bar, and then select “Open Network Preferences.”
Here, click “Advanced.”
Under the “Wi-Fi” tab, scroll through and select the network you want your Mac to forget, and then click the minus sign (-). Tip! You can select multiple at a time by pressing and holding the command key.
If you want to keep the password for that network, but not have your machine auto-connect, you can simply uncheck the box.
Click “OK” in the Wi-Fi screen.
Click “Apply” in the Network window to save your changes.
Your Mac has now forgotten the Wi-Fi network. If you want, you can click the Wi-Fi icon in the menu bar to log in to the network again.