MovieTexture.Stop

public void Stop ();

Description

Stops playing the movie, and rewinds it to the beginning.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public MovieTexture movTexture;
    void Start() {
        GetComponent<Renderer>().material.mainTexture = movTexture;
        movTexture.Play();
    }
    void Update() {
        if (Input.GetKeyDown(KeyCode.Space))
            movTexture.Stop();
        
    }
}