Click “Settings” on Menu in the top menu bar
Click on "Sites" right beside "Global settings"
Click the Settings icon for the site you want to go live/shut off (2nd Icon in the line)
See image here:
https://www.screencast.com/t/aqFmKqSvTaSB
The very first option in the "General" tab is "Enable/Disable Chat"
Click a radio button.
Click "save"