Disabling function of a key

So I need I'm trying to get a gameObject to jump, which I succeeded in, but its 50% of the way there to completion. I can get it to jump once and not get it to go up when I hold "Space", but if I spam tap space it'll keep going up. I think I either have a coding error somewhere, or I am just missing a piece of code somewhere. Could I please get someone to point out my error? Thanks! Here's my code: using UnityEngine; using UnityEngine.UI; using System.Collections;

 public class PlayerController : MonoBehaviour
 {
     public GameObject Player;
     public float speed;
     public Text countText;
     public Text winText;
     public float jumpHeight = 200f;
     public float bounceHeight = 200f;
 
     private Rigidbody rb;
     private int count;
 
     void Start()
     {
         rb = GetComponent<Rigidbody>();
         count = 0;
         SetCountText();
         winText.text = "";
     }
 
     void Update()
     {
         RaycastHit hit;
         Ray bouncingRay = new Ray(transform.position, Vector3.down);
 
         if (Physics.Raycast(bouncingRay, out hit, bounceHeight))
         {
             if (hit.collider.tag == "Ground" && Input.GetKeyDown("space"))
                 {
                     rb.AddForce(Vector3.up * jumpHeight);
                 }
             else
                 {
                 //need to disable key here
                 }
         }
     }
 
     void FixedUpdate()
     {
         float moveHorizontal = Input.GetAxis("Horizontal");
         float moveVertical = Input.GetAxis("Vertical");
 
         Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
 
         rb.AddForce(movement * speed);
 
     }
 
     void OnTriggerEnter(Collider other)
     {
         if (other.gameObject.CompareTag("Pick Up"))
         {
             other.gameObject.SetActive(false);
             count = count + 1;
             SetCountText();
         }
     }
 
     void SetCountText()
     {
         countText.text = "Count: " + count.ToString();
         if (count >= 8)
         {
             winText.text = "You Win!";
         }
     }
 }