If they are your networks you'd have to disable the radio/SSIDs in the modem/router/gateway, or otherwise unplug the device (no power) that are broadcasting the SSIDs.
If they are neighboring networks, theres nothing you can do.
If you are trying to remove saved credentials/settings for your own currently broadcasting SSIDs, you can always run the Network connection reset (Settings/System/Advanced system settings/Network connection reset).
