There is no way to completely turn off the light from the popup flash "Master" as the slave flash is only triggered by optical light. There is no radio transmitter or receiver.
Even when set to OFF the popup flash will still emit visible light in order to communicate with the slave. The amount of light emitted will be very low and should not be noticeable in the photo unless you are shooting a very reflective surface or very close to the subject.
One work around is block the popup flash with a small reflector, or even your hand, so that the light is reflected up or away but still able to reach the slave flash.