Public WiFi is becoming increasingly common these days, of course not all networks are open. Even the ones that appear open, frequently require login credentials. Because of this, it doesn’t make sense to have your iPhone or iPad constantly notifying you of nearby WiFi hotspots. If you find it annoying, here’s how to turn it off.
First go to Settings >> WiFi.
Switch Ask to Join Networks from On to Off.
Turning this option off will only stop iOS from asking you what WiFi network to join. It will still connect to previously joined networks automatically. If there is no known Wi-Fi network nearby, you’ll have to manually go into Settings >> WiFi and choose one. But that’s less annoying than being constantly interrupted by a pop up box everywhere you go.