There are a few options to quickly stop your second display.
1) Easiest way is to just unplug your iDevice from your Mac. Duet will no longer use system resources on your Mac and you will return to one display.
2) Press the sleep button on your iDevice. This will disconnect your display and you can resume single display activity on your Mac.
3) Force quit duet on the iPad. Simply double click the home button and flick the duet up to quit it.
4) Quit duet on the mac in the status menu. This is the least convenient as it means duet is no longer running on your Mac (it uses almost zero resources when in standby waiting for your iPad) and you will have to remember to open duet both on your Mac and your iDevice when you want to use it again.