Character Wont Stop Moving

I'm scripting movement for a 3D model with an animation, and I thought I scripted it at line 33-36 to STOP moving after you let go of the 'W' key. It isn't working. If anyone can help, that would be nice. Thank you.

 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.Threading;
 using UnityEngine;
 
 public class KniteController : MonoBehaviour
 {
     float speed = 4;
     float rotSpeed = 80;
     float gravity = 8;
 
     Vector3 moveDir = Vector3.zero;
 
     CharacterController controller;
     Animator anim;
 
     void Start()
     {
         controller = GetComponent<CharacterController>();
         anim = GetComponent<Animator>();
     }
 
     void Update()
     {
         if(controller.isGrounded)
         {
             if(Input.GetKey(KeyCode.W))
             {
                 moveDir = new Vector3 (0, 0, 1);
                 moveDir *= speed;
             }
             if (Input.GetKeyUp(KeyCode.W))
             {
                 moveDir = new Vector3 (0, 0, 0);
             }
         }
 
         moveDir.y -= gravity * Time.deltaTime;
         controller.Move(moveDir * Time.deltaTime);
 
     }
 
 
 
 }