How do I forget neighbours' (never-joined) Wi-Fi networks?
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.