unity - How can I prevent the balls from overlapping?

I am making a ball rolling game controlled by tilting an accelerometer.

In my game multiple balls overlap, so they end up showing as only one ball. How can I solve this problem, so balls do not overlap?

I have write this script:

public class Accelerometer : MonoBehaviour {

    public float Speed=0.5f;
    public float Speedy = 8f;
    public float HorizantalMin;
    public float HorizantalMax;
    public float VerticalMin;
    public float VerticalMax;

    // Use this for initialization
    void Start () {


    }

    // Update is called once per frame
    void Update () {
        float Horizental = Input.acceleration.x*Speed;
        float UP = Input.acceleration.y*Speedy;
        transform.Translate (Horizental, 0f,UP*Time.smoothDeltaTime);


        this.transform.position = new Vector3 (Mathf.Clamp (this.transform.position.x, HorizantalMin, HorizantalMax), this.transform.position.y, this.transform.position.z);

        this.transform.position = new Vector3 (this.transform.position.x, this.transform.position.y,Mathf.Clamp (this.transform.position.z, VerticalMin,VerticalMax));
    }
}