Stopping an object immediately

Hi I'm new to making games and I don't know how make an object stop immediately. I already did some research but nothing helped me. Also, do you guys maybe have some advise to make the script's code a bit more clean? Thanks in advance :)

 void Start()
 {
     rb = GetComponent<Rigidbody2D>();
 }

 void Update()
 {
     if (Input.GetKey(KeyCode.UpArrow))
     {
         MoveUp();
     }

     if (Input.GetKey(KeyCode.DownArrow))
     {
         MoveDown();
     }
 }

 void MoveUp()
 {
     float y = Input.GetAxisRaw("Horizontal");
     float upwardspeed = y + speed;
     rb.velocity = new Vector2(0, upwardspeed);
 }

 void MoveDown()
 {
     float y = Input.GetAxisRaw("Horizontal");
     float downwardspeed = y - speed;
     rb.velocity = new Vector2(0, downwardspeed);
 }