'Remove expired instances of ``RegistrationProfile`` and their
associated ``User``s.
Accounts to be deleted are identified by searching for
instances of ``RegistrationProfile`` with expired activation
keys, and then checking to see if their associated ``User``
instances have the field ``is_active`` set to ``False``; any
``User`` who is both inactive and has an expired activation
key will be deleted.
It is recommended that this method be executed regularly as
part of your routine site maintenance; this application
provides a custom management command which will call this
method, accessible as ``manage.py cleanupregistration``.
Regularly clearing out accounts which have never been
activated serves two useful purposes:
1. It alleviates the ocasional need to reset a
``RegistrationProfile`` and/or re-send an activation email
when a user does not receive or does not act upon the
initial activation email; since the account will be
deleted, the user will be able to simply re-register and
receive a new activation key.
2. It prevents the possibility of a malicious user registering
one or more accounts and never activating them (thus
denying the use of those usernames to anyone else); since
those accounts will be deleted, the usernames will become
available for use again.
If you have a troublesome ``User`` and wish to disable their
account while keeping it in the database, simply delete the
associated ``RegistrationProfile``; an inactive ``User`` which
does not have an associated ``RegistrationProfile`` will not
be deleted.'

def delete_expired_users(self):

    for profile in self.all():
        try:
            if profile.activation_key_expired():
                user = profile.user
                if (not user.is_active):
                    profile.delete()
                    user.delete()
        except UserModel().DoesNotExist:
            profile.delete()
