Go to Wi-Fi settings and click the "i" beside the problematic network.

You have two options that should solve the problem. Either:

a) Tap "forget this network", or

b) Toggle "auto-join" to off

This is my solution too. I rarely use "public" wifi, but if I do, I ALWAYS turn "auto-join" off.

Also, from time to time, I go in and reset my network settings, which erases any networks I've forgotten to forget.

The only way I’ve managed to avoid my iPhone connecting to random wifi is to go to settings:wifi and set “ask to join” to true. And now it just bugs me with prompts a hundred times every train ride.

Ifaik there’s no other way to stop autoconnecting to wifi that matches certain conditions, and it drives me up the wall.

Have you tried leaving "ask to join" off, and forgetting the problematic network? If you rarely if ever need to connect to it, it's better that you don't save it to your list of known networks.

The other way I see from the other comment is to view that SSID in your wifi menu, click the (i) button next to it, and toggle "auto-join".

I tried this initially but new wifi networks kept springing up with such frequency that the whole thing became a game of whack-a-mole. Maybe they rotate names frequently here? I might give it another shot though. There can’t be that many wifi networks between here and Roppongi...

My iPhone only ever connects to networks I've used -- never unknown open networks. I know it's possible to enable automatic asking (I think, via "ask to join network"?), but that seems like an annoying feature.

Yeah, I think it’s trying to connect to networks that are somehow “partners” with my provider, and for some reason there seems to be hundreds of these scattered around Tokyo.

>The only way I’ve managed to avoid my iPhone connecting to random wifi is to go to settings:wifi and set “ask to join” to true. And now it just bugs me with prompts a hundred times every train ride.

Ever considered just turning wifi off?

I do this occasionally. I just wish Apple made it easier to turn off and on permanently - as is right now you have to go into the settings page and flick a switch.

Though, given my terrible memory this particular solution sometimes comes back to bite me in the bum: Usually when I turn off wifi I forget to turn it back on again until I’m at work and I’ve already downloaded a few hundred megs of music, all the whole stupidly ignoring the 4g connection icon.