Ignore collision effect while keeping the colliders

In my game there are many objects performing projectile motion (using gravity and AddFore)and the player have to shoot balls to it to destroy it.

There is a special kind of object (stone) that flicks away the approaching ball when contact.

I am using OnCollisionEnter and Collision.contacts[0].normal to perform reflection motion of the ball and it works well.

But the side effect is that the stone will be "pushed" or drop immediately after collision. How do I remove this effect and make it to perform the original projectile motion?