//Please go to functions/theme-shortcodes.php file and find line 1101 which looks like:

$output .= '<iframe class="scale-with-grid" width="'. $width .'" height="'. $height .'" src="http://www.youtube.com/embed/'. $video .'" frameborder="0" allowfullscreen></iframe>'."\n";

//and change it to below one:

$output .= '<iframe class="scale-with-grid" width="'. $width .'" height="'. $height .'" src="http://www.youtube.com/embed/'. $video .'?rel=0" frameborder="0" allowfullscreen></iframe>'."\n";