With iOS the new UX seems a little bad. I have AT&T. If I go to the Wifi settings and tap "Edit", and then go to the AT&T Passpoint managed network, tap the "i" button, and disable auto-join, then back out of the detail view, the save button is still disabled. However, if I just tap the back button out of the managed view, it still saves.
So, anyway, at least for AT&T you can definitely disable the auto-join at least.