How to disable a script in another without Public GameObject?

I just want to disable a script that's in my player-manager. But because my enemies spawn through set active the public game object just isn't working. Can I just manually pick it out through a specific GameObject in the script?

 public float lookRadius = 40f;

 //public Casting stop;

 Transform target;
 UnityEngine.AI.NavMeshAgent agent;

 Rigidbody theRigidBody;

 void Start(){
     target = PlayerManager.instance.player.transform;
     agent = GetComponent<UnityEngine.AI.NavMeshAgent>();
 }


 void Update(){
     float distance = Vector3.Distance (target.position, transform.position);
     if (distance <= lookRadius)
     {
         agent.SetDestination (target.position);

         if (distance <= agent.stoppingDistance)
         {
             FaceTarget ();
         }

         if (distance < 10f) // or some distance
         {
             //gameObject.GetComponent<Casting>().enabled = false;
             Debug.Log("nearby heyy");
         }
     }

 }

 void FaceTarget()
 {
     Vector3 direction = (target.position - transform.position).normalized;
     Quaternion lookRotation = Quaternion.LookRotation (new Vector3 (direction.x, 0, direction.z));
     transform.rotation = Quaternion.Slerp (transform.rotation, lookRotation, Time.deltaTime * 5f);
 }