Forget should remove all stored credentials. I don't understand what more you want, because as far as I can see that does put it in the same state as if you'd not used it (as far as the phone is concerned - the network's own logs will show you used it, but that's nothing you can do anything about).
If you don't want to see it in your list of networks then long press on the name and block notifications, but that wouldn't be how it would behave for a network you'd simply not used.