Depends what you're talking about. Do you mean for the flight controls to actually slam all the way to stop, then center, then slam to stop, etc., violently slamming the aircraft around? (Terrible for fuel economy by the way, and hard on everything else).
Or do you mean to use PWM to select an intermediate position? E.g. 50% duty cycle gives you stable 15 degree rudder?
The trick is to put damping in the operating mechanism, and PWM at a frequency fast enough for the damping to do its job: before the surface physically gets a chance to move, the PWM reverses. The PWM is too fast to wobble the surfaces.
Then also either add some spring resistance so deflection will be predictable for a given duty cycle, or have an analog position sensor educating the PWM as to its effects. Air resistance varies with speed, and without something to equalize it, surfaces at a given duty cycle will deflect different amounts for different speeds. Of course, that could be made a "feature": as DJ319 points out, you actually want less deflection at higher speeds.