Some members still register with old joomla registration form !

Hi,
our defaut registration component is Easysocial, all new members use the ES forms to join us except some ones (0.5%) still use the joomla registration component, who we can know that ? because we receive some time the defaut mail sent by joomla (Account details for).

when those members register with it we get a profil wich is not conform to our easysocial profil.
any wah to check how thoses members are getting the old registration method ,

regards