how do i stop automatic firing?

You need to understand a little more about coding in general and for unity in specific. What your code does right now is calling the Start function which is called once on the first frame the object is active which goes through the Shoot() function which shoots one bullet exactly. You haven't actually implemented any system to use input from user or firing at a specific rate.

You need to give us more information about how you want the triggering of the firing to occur, but as for the firing itself you need to implement a delayed firing system which actually uses the fire rate variable. Something like this:

  public Transform firePoint;
  public GameObject bulletPrefab;
  public float fireRate; //addition
  private float nextfire; //addition
  
  void Update()
  {
     if (Time.time >= nextfire){
         Shoot();
     }
  }
  
  public void Shoot()
  {    
        nextfire = Time.time + fireRate;
        Instantiate(bulletPrefab, firePoint.position, firePoint.rotation);
        Debug.Log("Bang");
        //Instantiate (flash, flashSpawn.position, flashSpawn.rotation); //addition
       //    GunShotSound.Play (); //addition
  }

remove the Start method completly

I solved auto fire myself. I re-added;

if (Input.GetButtonDown("Fire1") && Time.time > nextfire)

to start and made start public and connected the fire button to it. Thanks for your help anyway.

As for fire rate, I don't use it in any other script and the only other script associated with firing is attached to the bullet and that covers speed, what it's hit and destroying it after 4 frames

 public float speed = 20f;
 public Rigidbody rb;

 // Use this for initialization
 void Start()
 {
    rb.velocity = transform.forward * speed;
 }

 void OnTriggerEnter (Collider hitInfo)
 {
    Debug.Log(hitInfo.name);
 }

 // Update is called once per frame
 void Update()
 {
     Destroy(gameObject, 4.0f);
 }