Disable bluetooth keyboard for second mac
On the second Mac, under System Preferences > Bluetooth > "Paired Devices" > right click (or control + click) the device (keyboard) > Remove. You must be connected to bluetooth to remove the device. 

Under "Advanced" uncheck "Open Bluetooth Setup Assistant at Startup if No Keyboard is Connected"

Subsequently, you can disable Bluetooth altogether in the same pane > "Turn Bluetooth Off."