Destroying gameobject

Im new to unity and wanted to make a simple game where i have to shoot some cubes. But i run into 2 problems. problem 1) If i shoot at a cube and keep shooting it, my score increases every frame and not just 1 time. problem2) i wanted to solve this and tried to destroy the cube after it was hit. But i dont know how i can destroy the cube not the bullet.

Here my code

 using UnityEngine;
 using System.Collections;
 
 public class LaserScript : MonoBehaviour {
 
     LineRenderer line;
     public int weaponPower = 10;
     public int score = 0;
 
 
     void Start () 
     {
         line = gameObject.GetComponent<LineRenderer> ();
         line.enabled = true;
         gameObject.GetComponent<LensFlare> ().enabled = false;
 
         Screen.lockCursor = true;
     }
     
 
     void Update () 
     {
         if (Input.GetButtonDown ("Fire1")) 
         {
             StopCoroutine("FireLaser");
             StartCoroutine("FireLaser");
         }
         if (score == 10) 
         {
             Debug.Log ("You Won");
         }
         
     }
     IEnumerator FireLaser()
     {
         line.enabled = true;
         gameObject.GetComponent<LensFlare> ().enabled = true;
 
         while (Input.GetButton("Fire1")) 
         {
             line.renderer.material.mainTextureOffset = new Vector2(0,Time.time);
             Ray ray = new Ray(transform.position,transform.forward);
             RaycastHit hit;
 
             line.SetPosition(0,ray.origin);
 
             if(Physics.Raycast(ray,out hit,3000))
             {
                 line.SetPosition(1,hit.point);
                 if(hit.rigidbody)
                 {
                     hit.rigidbody.AddForceAtPosition(transform.forward*weaponPower,hit.point);
 
                     score++;
 
 
 
                     Debug.Log(score);
                 }
             }
             else
                 line.SetPosition(1,ray.GetPoint(3000));
 
             yield return null;
         }
 
         line.enabled = false;
         gameObject.GetComponent<LensFlare> ().enabled = false;
     }
 }

Hopefully someone can help me. And sorry for my bad english ;)