Unsuccessfully trying to clamp my player object

Hi I am new to unity and am finding it very fun. However I've come across this issue and I I've tried a lot of fiddling around with this code but I just can't seem to stop the player from moving off of the screen.

What am I doing wrong here? I am making a top down facing arcade shooter. I don't want the spaceship to go beyond the edge of the screen (when in game mode).

public class PlayerController : MonoBehaviour {

     public float xMin, xMax, zMin, zMax;
     private Rigidbody rb;
     public float speed;
 
 
     // Use this for initialization
     void Start () {
         rb = GetComponent<Rigidbody> ();
     }
     
     // Update is called once per frame
     void FixedUpdate () {
 
         float moveHorizontal = Input.GetAxis("Horizontal");
         float moveVertical = Input.GetAxis ("Vertical");
 
         Vector3 movement = new Vector3(moveHorizontal,0.0f,moveVertical);
         GetComponent<Rigidbody>().AddForce (movement);
 
         rb.position = new Vector3 (Mathf.Clamp (GetComponent<Rigidbody>().position.x, xMin, xMax), 0.0f, Mathf.Clamp (GetComponent<Rigidbody>().position.z, zMin, zMax));
 
     }
 
 
 
 }

Thanks for the help.