Question How do I stop the player from moving mid-air using this script

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class Movement : MonoBehaviour
6. {
7.     private Vector3 PlayerMovementInput;
8.     [SerializeField] private LayerMask FloorMask;
9.     private Transform FeetTransform;
10.     private Rigidbody PlayerBody;
11.     [Space]
12.     private float moveSpeed = 0f;
13.     [SerializeField] private float Speed = 5f;
14.     [SerializeField] private float SprintSpeed = 7f;
15.     [SerializeField] private float JumpForce;
16.     private bool isGrounded;
17.  
18.     private void Start()
19.     {
20.         FeetTransform = this.transform.GetChild(1).transform;
21.         PlayerBody = GetComponent<Rigidbody>();
22.     }
23.  
24.     void Update()
25.     {
26.         isGrounded = Physics.CheckSphere(FeetTransform.position, 0.1f, FloorMask);
27.  
28.         PlayerMovementInput = new Vector3(Input.GetAxis("Horizontal"), 0f, Input.GetAxis("Vertical"));
29.         MovePlayer();
30.         if (Input.GetKeyDown(KeyCode.LeftControl))
31.         {
32.             Vector3 MoveVector2 = transform.TransformDirection(PlayerMovementInput) * SprintSpeed;
33.             PlayerBody.velocity = new Vector3(MoveVector2.x, PlayerBody.velocity.y, MoveVector2.z);
34.         }
35.     }
36.  
37.     private void MovePlayer()
38.     {
39.         if (Input.GetKey(KeyCode.LeftControl))
40.         {
41.             moveSpeed = SprintSpeed;
42.         }
43.         else
44.         {
45.             moveSpeed = Speed;
46.         }
47.  
48.         Vector3 MoveVector = transform.TransformDirection(PlayerMovementInput) * moveSpeed;
49.         PlayerBody.velocity = new Vector3(MoveVector.x, PlayerBody.velocity.y, MoveVector.z);
50.  
51.         if (Input.GetKeyDown(KeyCode.Space))
52.         {
53.             if (isGrounded)
54.             {
55.                 PlayerBody.AddForce(Vector3.up * JumpForce, ForceMode.Impulse);
56.             }
57.  
58.         }
59.     }
60. }
61.