I don't know where the "automatically join networks" setting is?
"Ask to Join Networks" - If you turn this off the iPhone won't prompt you when it finds an unknown network (i.e. one you have never connected to before, or forgotten).
If "Wi-Fi" is ON the phone will search and connect automatically to any network which it already knows (i.e. a network you have previously connected to)
The only way to stop the phone searching for any network at all is to turn Wi-Fi OFF
Hopefully that covers your question.