public function deleteUser($username)
    {
        /** @var UserInterface|EloquentUser $user */
        if ( ! ($user = $this->sentinel->findByCredentials(['email' => $username]))) {
            return false;
        }

        // The super admin may not be deleted.
        if ($user->isAdmin()) {
            return false;
        }

        return (bool) $user->delete();
    }