<iframe width="560" height="315" src="https://www.youtube.com/embed/mKt-MDwtVB4" frameborder="0" allowfullscreen></iframe>


  <button class="fancybox-close">Stop video please</button>

body {width:60%;margin:3em auto;}
footer {border-top:1px solid #ccc;padding:1em 0;}

    // Finds all iframes from youtubes and gives them a unique class
    jQuery('iframe[src*="https://www.youtube.com/embed/"]').addClass("youtube-iframe");


    jQuery(".fancybox-close").click(function() {
      // changes the iframe src to prevent playback or stop the video playback in our case
      $('.youtube-iframe').each(function(index) {
        $(this).attr('src', $(this).attr('src'));
        return false;
      });
      
//click function
    });