How to delete an old wi-fi network
How do I DELETE an old wi-fi network?
MacBook Pro with Retina display
How do I DELETE an old wi-fi network?
MacBook Pro with Retina display
I'm assuming you do not want the network to be a default selection
System Preferences -> Network -> Advanced -> Wi-Fi -> Preferred Networks: -> [-]
Just select the network and click the minus [-] button.
If you are referring to networks that show up in your Wi-Fi menu bar list, you cannot do that as long as the owner of the network is operating their WiFi base station and you are in range.
If you happen to be the owner of the old WiFi base station, turn off the power, or use the WiFi base stations admin interface to turn off the WiFi radio (if it has that option), or change its name if you just want to get rid of the old name.
If none of my guesses are correct, could you please provide more details on what it is you are trying to get rid of?
How to delete an old wi-fi network