Stop Object From Moving After Key Release

I am using the following code to make an object move left/right. The object has a circle Collider 2D and a Rigidbody 2D attached to it. The Collider 2D has a Physics2D Material with Bounce = 0 and Friction = 0.

When use the arrow keys to move left/right and then let go of the keys, the object wants to keep moving left/right. I want it to stop as soon as I let go of the keys.

Sometimes when I move in another direction as soon as I let go of the keys it starts moving the other way.

What is causing this and how can I get it to stop when I let go of the directional keys?

 using UnityEngine;
 using System.Collections;

 public class Move : MonoBehaviour {

     // Update is called once per frame
     void Update () {
         if(!renderer.isVisible){
             Debug.Log("Can't see me!");
         }
         this.transform.Translate(Input.GetAxis("Horizontal") * Time.deltaTime * 15,0,0);
     }
 }