How to not load comments form on post preview?

I took a quick peek at the disqus plugin. This works in disabling the option before the plugin decides to print out the form.

add_filter( 'pre_option_disqus_active', 'wpse_conditional_disqus_load' );
function wpse_conditional_disqus_load( $disqus_active ) {
  if( is_preview() ){
    return '0';
  }

  return $disqus_active;

}

You could also try something like this (not tested)

add_filter( 'the_content', 'wpse_load_disqus');
function wpse_load_disqus( $content ){
  if( is_preview() ){
    return $content;
  }

  if( is_singular() ) { // displays on all single post types. use is_single for posts only, is_page for pages only

    $content .= ?>
      // You disqus script here
    <?php ;
  }

  return $content;

}