for (var num = balls.Count -1; 0 < balls.Count; num--) {
        balls.RemoveAt (num);

for (var num = balls.Count -1; 0 < balls.Count; num--) {
        Destroy (balls[num]);
    }

for (var num = balls.Count -1; 0 < balls.Count; num--) {
        Destroy (balls[num]); //オブジェクトの削除
        balls.RemoveAt (num); //リストの削除
    }