class UserProfile(models.Model):
    description = models.TextField()

class Channel(models.Model):
    subscribed = models.ManyToManyField(UserProfile,
                                        related_name="subscribed")

c = Channel.objects.get(pk=1)
subscribed = c.subscribed_set.all()
not_subscribed = UserProfile.objects.exclude(pk__in=[s.pk for s in subscribed])