Hi,
I assume you have checked your Motor directions and mounted your Copter roughly as described here: http://px4.io/docs/1595-2/
If you try to rotate Yaw does the sound of the copter change?
One way to prevent the Props from spinning would be to lower the lower limit of PWM on Armed Mode. By Default this should be 1000.
This has been discussed already here: Can i stop motor spinning when arm