I had a similar problem recently (see How do I avoid checking for updates for a specific module?) and managed to get around it using hook_update_projects_alter(). 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.
