how do I remove a WiFi network from the list of those available on either my iPhone or iPad?

Hi. Unfortunately, you can only forget a network when connected to it. Otherwise, you can use Settings > General > Reset > Reset Network Settings to forget everything. Then, you'll need to reconnect to the ones you use.

Your welcome.

Regarding your question, you can only do so if you are logged in to that network.

Go to Settings > Wi-Fi > select the circle with the i in it and then click on Forget This Network.

You cannot remove the available networks as it will show all. However you can forget a network to prevent your devices from connecting automatically if they have connected to it in the past.

You are welcome.