Invalid WI-FI Networks showing in "WI-FI Networks" list
While all networks you've connected to in the past are stored in a file called /data/misc/wifi/wpa_supplicant.conf, networks "just seen" around (but never connected to) are IMHO not stored to any file, but just "cached" in memory. So a simple trick to get rid of them is to enter airplane-mode for a couple of seconds (or a minute if seconds are not enough). With WiFi being disabled, the system should see no reason to "stick" to that "cached" information, and thus "forget" the unknown networks. As soon as you return from airplane mode, it would need to "rebuild" its cache again – but those "stickies" should be gone for good (until next time, maybe, but then simply repeat the action).