Try deleting the device via the RhoConnect admin UI.
Deleting the user will also delete all of its devices and clear up the user's data.
So, the next time that device syncs - it should get the 'unknown client' error. The only way to override this would be to login from the device again - and that wouldn't be possible if the password has changed.
If you do not want to delete the user - you can delete just the device. It will lead to the same consequences - next time the deleted device will try to sync it will get the "unknown client" error.