Locking onto a enemy until its dead

I made a script which looks at an enemy and shoots at it as long as i hold the mouse button onto it, but i cant figure out how to make it lock onto the enemy only by clicking once and then let it go when the enemy dies.

Player code:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.AI;
 
 public class PlayerController : MonoBehaviour
 {
     public Camera cam;
     public NavMeshAgent agent;
 
     public float damage = 10f;
     public float range = 100f;
     public float rotspeed = 10f;
     public GameObject Gun;
     public Quaternion targetRotation;
     public float impactForce = 10f;
     public float firerate = 0f;
     private float nextTimeToFire = 0f;
     public ParticleSystem flash;
     public GameObject impact;
     void Start()
     {
 
     }
 
     // Update is called once per frame
     void Update()
     {
         if (Input.GetMouseButton(0) )//&& Time.time >= nextTimeToFire)
         {
             Ray ray = cam.ScreenPointToRay(Input.mousePosition);
             RaycastHit hit;
 
             if (Physics.Raycast(ray, out hit))
             {
                 if (hit.transform.gameObject.layer == LayerMask.NameToLayer("Default"))
                 {
                     agent.SetDestination(hit.point);
                 }
                 else if (hit.transform.gameObject.layer == LayerMask.NameToLayer("Enemy"))
                 {
                         targetRotation = Quaternion.LookRotation(hit.point - transform.position);
                         transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, rotspeed * Time.deltaTime);
 
                         Shoot();
                 }
             }
         }
 
 
     }
     void Shoot()
     {   
         if (Time.time >= nextTimeToFire)
         {   
             flash.Play();
             nextTimeToFire = Time.time + 1f / firerate;
             RaycastHit hit;
             if (Physics.Raycast(Gun.transform.position, Gun.transform.forward, out hit, range))
             {
                 Debug.Log(hit.transform.name);
                 Target target = hit.transform.GetComponent<Target>();
 
                 if (target != null)
                 {
                     target.TakeDamage(damage);
                 }
                 if (hit.rigidbody != null)
                 {
                     hit.rigidbody.AddForce(-hit.normal * impactForce);
                 }
 
                 GameObject impactGO = Instantiate(impact, hit.point, Quaternion.LookRotation(hit.normal));
                 Destroy(impactGO, 1f);
             }
         }
     }
 }
 

Enemy code:

 public class Target : MonoBehaviour
 {   
     public float hp = 50f;
     public void TakeDamage (float amount)
     {
         hp -= amount;
         if (hp <= 0f)
         {
             Die();
         }
     }
     void Die()
     {
         Destroy(gameObject);
     }