How does one stop the "Select A Wireless Network" prompt on iOS 10 Beta?
Firstly, make sure to report the issue using Feedback Assistant. Then, try toggling the setting in Settings, as it might fix the issue. If it doesn't, try restarting the iPhone. Then you could also try backing up the iPhone, erasing all data on it, then restoring the backup. But, if that still doesn't fix the issue, and it really bothers you, you can always downgrade to iOS 9.3.2.