How do I forget neighbours' (never-joined) Wi-Fi networks? My WiFi can see 12 networks belonging to neighbours, locked and password protected, some with silly names. Network/Wi-Fi/Advanced allows me to remove names from the list, but only for networks I have joined. How can I remove the others?
Network > Wi-Fi > Advanced allows you to remove preferred networks. Preferred means that they are recognized (i.e. previously-connected-to) networks and you can set a priority order should multiple networks be available simultaneously. These networks will still appear in the list of available networks when they are nearby after removal from Preferred.

There is no way to remove network SSIDs from the list of currently-available networks.