Disable Author pages in Wordpress

 Open your theme’s functions.php and put this in:

/* Kill attachment, search, author, daily archive pages */
add_action('template_redirect', 'bwp_template_redirect');
function bwp_template_redirect()
{
 global $wp_query, $post;

 if (is_author() || is_attachment() || is_day() || is_search())
 {
  $wp_query->set_404();
 }

 if (is_feed())
 {
  $author  = get_query_var('author_name');
  $attachment = get_query_var('attachment');
  $attachment = (empty($attachment)) ? get_query_var('attachment_id') : $attachment;
  $day  = get_query_var('day');
  $search  = get_query_var('s');

  if (!empty($author) || !empty($attachment) || !empty($day) || !empty($search))
  {
   $wp_query->set_404();
   $wp_query->is_feed = false;
  }
 }
}

The snippet above will effectively disable attachment pages, search page, author pages and daily archive pages. Automatically generated feeds for those pages will also be disabled.

You can find more conditional tags at: http://codex.wordpress.org/Conditional_Tags

Source: http://betterwp.net/wordpress-tips/disable-some-wordpress-pages/