2D Charachter infinite jumping

hi, just a simple question from a noob, how do I get this to stop jumping infinitely?

using UnityEngine; using System.Collections;

public class PlayerController : MonoBehaviour {

  [HideInInspector]
  public bool isFacingRight = true;
  [HideInInspector]
  public bool isJumping = false;
  [HideInInspector]
  public bool isGrounded = false;
  public GameObject thing;
  public GameObject floor;
  public GameObject platform;
  public float maxSpeed = 7.0f;
  public float jumpForce = 650.0f;
  public Transform groundCheck;
  public LayerMask groundLayers;
  private float groundCheckRadius = 0.2f;
  void Start()
  { }
    
  void Update()
  {
      if (Input.GetButtonDown("Jump"))
      {
          if (isGrounded == true)
          {
              this.GetComponent<Rigidbody2D>().velocity = new Vector2(GetComponent<Rigidbody2D>().velocity.x, 0);
              this.GetComponent<Rigidbody2D>().AddForce(new Vector2(0, jumpForce));
          }
          if (floor == thing)
          {
              this.GetComponent<Rigidbody2D>().velocity = new Vector2(GetComponent<Rigidbody2D>().velocity.x, 0);
              this.GetComponent<Rigidbody2D>().AddForce(new Vector2(0, jumpForce));
          }
      }
  }
  void FixedUpdate()
  {
      isGrounded = Physics2D.OverlapCircle
        (groundCheck.position, groundCheckRadius, groundLayers);
      float move = Input.GetAxis("Horizontal");
      this.GetComponent<Rigidbody2D>().velocity =
        new Vector2(move * maxSpeed, GetComponent<Rigidbody2D>().velocity.y);
      if ((move > 0.0f && isFacingRight == false) || (move < 0.0f && isFacingRight == true))
      {
          Flip();
      }
  }
  void Flip()
  {
      isFacingRight = !isFacingRight;
      Vector3 playerScale = transform.localScale;
      playerScale.x = playerScale.x * -1;
      transform.localScale = playerScale;
  }