Samsung’s Android skin won’t let you tell it to forget a saved WiFi network unless it can see it right now. If it’s present, sure. If it’s out of range, it won’t let you tell it that no, you really don’t want it to automatically connect the next time you’re in range.
This is both annoying (your Galaxy S7 or Note 5 is going to keep looking for those networks all. the. time.) and a security risk (imagine someone sets up a rogue hotspot called “Starbucks WiFi” and you happen to park your car* or sit on a bench within range of it).
Note that the stock Android settings do allow you to fix this, with a Saved Networks section in the WiFi config. Samsung deliberately removed the feature** for some reason.
Apparently it used to be possible to remove saved networks using a third-party app, but new security protections in Marshmallow prevent that. (Ironic, that.)
- List all saved networks by going to Settings → Data Usage → More → Restrict networks. (This doesn’t let you remove them, just limit background transfers on them.) Take a screenshot if you have to.
- Remove the ones you don’t want anymore by tediously renaming your own WiFi hotspot to match each in turn [edit: you also need to match the security type (open vs WPA2)], removing them one by one in the regular WiFi settings, then renaming your hotspot back to its normal SSID.
It’s a pain, but at least it’s possible.
Update May 2017: The Android 7 update finally restores this capability directly in the Settings app, at least on the Galaxy Tab S2. You can now go to
Settings → Connections → Wi-Fi → Advanced → Manage networks
to remove saved networks of just turn off auto-reconnect on a case-by-case basis (so you can keep saved passwords). But for older Android versions, we’re still stuck doing it the long way.
*I once stopped at a Coffee Bean and left my tablet in the car. I don’t remember why I pulled it out when I got back in the car, but it had connected to the WiFi while I was grabbing my coffee. It couldn’t actually load anything but the login page because it was the real hotspot…but if it had been a fake hotspot, they could have intercepted or modified any non-encrypted traffic going on in the background.
**At best, Samsung forgot to include it when they wrote their own settings app years ago.