Stopping a 2D rigid body.

Hello everyone! And thank you for reading this. So my problem is hopefully simple. I have come up with a basic movement system for a ship. When I press up it goes forward, when I press left and right it changes its rotation. My only problem is down. When I press down I want the ship to stop, but I don’t know how to do that. For now I set it to go backwards. Here is my code if you can help it would be very much appreciated.

using UnityEngine;
using System.Collections;

public class move : MonoBehaviour {
		public float speed;
		public float rotationSpeed;
		// Use this for initialization
		void Start () 
		{

		}
		
		// Update is called once per frame
		void Update () 
		{
			if (Input.GetKey ("right")) 
			{
				transform.Rotate(Vector3.back * rotationSpeed, rotationSpeed * Time.deltaTime);
			}
			
			if (Input.GetKey ("left")) 
			{
				transform.Rotate(Vector3.forward * rotationSpeed, rotationSpeed * Time.deltaTime);
			}
			
			if (Input.GetKey ("up")) 
			{
				rigidbody2D.AddForce(transform.up *  speed);
			}
			
			if (Input.GetKey ("down")) 
			{
				rigidbody2D.AddForce(transform.up *  -speed);
				
			}
		}
	}

Try this. Is is making the rigidbody kinematic when is it stoped and not when it’s moving.

using UnityEngine;
using System.Collections;

public class move : MonoBehaviour {
	public float speed;
	public float rotationSpeed;
	// Use this for initialization
	void Start () 
	{
		
	}
	
	// Update is called once per frame
	void Update () 
	{
		if (Input.GetKey ("right")) 
		{
			rigidbody2D.isKinematic = false;
			transform.Rotate(Vector3.back * rotationSpeed, rotationSpeed * Time.deltaTime);
		}
		
		if (Input.GetKey ("left")) 
		{
			rigidbody2D.isKinematic = false;
			transform.Rotate(Vector3.forward * rotationSpeed, rotationSpeed * Time.deltaTime);
		}
		
		if (Input.GetKey ("up")) 
		{
			rigidbody2D.isKinematic = false;
			rigidbody2D.AddForce(transform.up *  speed);
		}
		
		if (Input.GetKey ("down")) 
		{
			rigidbody2D.isKinematic = true;
		}
	}
}