I suspect a stepper motor used directly would not be suitable unless a single step is sufficient. And you don't have any control over how fast the motor moves with a single step - that just depends on the magnetism and the mass of the motor and whatever is connected to it.
If you are trying to use the rotational velocity (and the rotating mass) of the stepper then a stepper motor is no different from any other motor. And if the stepper motor is rotating quickly it almost certainly won't stop instantly.
My inclination would be to use a motor to "charge" a mechanical device that will deliver the impact - for example to pull back a piston against a spring and then have a separate mechanism to release the piston using the force of the spring to deliver the impact.