Phones do not auto connect to networks unless they are saved, have a third party application installed, or have some kind of device management profile installed. For iPhone, standard methods of disabling auto join or forgetting the network, deleting an offending app, or removing a profile will work. I previously posted instructions for removing a profile. There are too many versions or variations of Android based phones to explain instructions - please search a site like google for how to manage android WiFi profiles, delete apps or limit permissions, or remove android mobile device management profiles. Note that not all apps in android have an icon on the screen; you will need to remove any “hidden” background apps as well.
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