'Removes permission ``perm`` for an instance ``obj`` and given ``user``.'

def remove_perm(self, perm, user, obj):

    if (getattr(obj, 'pk', None) is None):
        raise Exception(('Object    %s    needs    to    be    persisted    first' % obj))
    filters = {'permission__codename': perm, 'permission__content_type': ContentType.objects.get_for_model(obj), 'user': user}
    filters['object_pk'] = obj.pk
    self.filter(**filters).delete()
