I had a similar problem recently (see http://drupal.stackexchange.com/questions/26079/how-do-i-stop-drupal-checking-for-updates-for-a-specific-module) and managed to get around it using [`hook_update_projects_alter()`][1]. You can use it to easily remove any module that you don't want to be part of the update process:

    function MYMODULE_update_projects_alter(&$projects) {
      $blacklist = array(
        'sharethis',
        // Any others you might want
      );

      foreach ($blacklist as $module) {
        unset($projects[$module]);
      }
    }

As Drupal no longer considers any module you put in the `$blacklist` array as one that needs to be updated, you should stop getting the security warning.

It would probably be a good idea to keep an eye on the progress of the bug in the ShareThis module so that you can remove this restriction when possible.

[1]: http://api.drupal.org/api/drupal/modules%21update%21update.api.php/function/hook_update_projects_alter/7