I want to keep syncing WiFi passwords through keychain, but I want to keep my iPhone off specific WiFi network, which I must use on my work Mac. How do I do this?
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.
On your iPhone, go to Settings > WiFi. Then tap the (i) button beside the network you don't want it to access anymore. At the top of your screen will be a blue "Forget This Network". Tap that and tap "Forget" to confirm your choice.
Your Mac should still remember the network and join it freely, but your iPhone should no longer join it.
If your iPhone keeps joining the network after doing the above, be sure to enable the option for "Ask to Join Networks" (at the bottom of the screen in Settings > WiFi).