When any user is registred on joomla, beside you disable email registration, the system is sending an e-mail to administrator for new user details; we need to avoid that send mail function.
Solution:
Acces to file "controller.php" on path "components/com_user" and edit the next line:
Replace:
UserController::_sendMail($user, $password);
With:
// UserController::_sendMail($user, $password);