How to disallow iPhone from joining specific WiFi network without turning off keychain sync and thus still allow to join it on Mac?
If you have admin control of the wifi network (or can contact who does), you could have your iPhone's MAC address blocked from the network with MAC Address filtering...

However this may (or may not, totally theoretical) impact your iPhone's battery life if the phone starts repeatedly trying to join the network when in range. Like I said, it might not, I'm not sure what it's behaviour would be if the network told it to sod off when joining :)

Depending on the network setup, when you tap the little (i) next to the network name, you can  turn off a option called "Auto-Join". So the keychain retains the network details, but does not join the network unless you tap on it. I think this is a setting enabled by RADIUS enabled networks or networks added to the device by a Profile though, I've never figured out why some networks show it and some do not.