Target single block with amended theme function?
you can use preprocess_block function as follows:

 function mytheme_preprocess_block(&amp;$variables) {
      if ($variables['block_id']-&gt;bid === 'target_block_id') {
       $users = $variables['users'];
      $title = $variables['title'];
      $items = array();

      if (!empty($users)) {
        foreach ($users as $user) {

          // $items[] = theme('username', array('account' =&gt; $user));

          $picture = theme_get_setting('toggle_comment_user_picture') ? theme('user_picture', array('account' =&gt; $user)) : '';
          $username = theme('username', array('account' =&gt; $user));
          $items[] = $picture . $username;
        }

      }
      return theme('item_list', array('items' =&gt; $items, 'title' =&gt; $title));
      } 
    }