If you have a #mac and you want to forget the open #wifi access points that your mac has joined, this handly little python script does the trick. It leaves alone any wifi that has a password. Only deletes the WiFi networks that have no password. If you use #iCloud keychain, this will have the lovely side-effect of deleting them from your #iPhone/#iPad at the same time. https://github.com/mubix/osx-wificleaner
You might also want to disable the captive network assistant on your mac:
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