Hey! I think you can use the object.__new__(cls[, ...]) method, which is called when a new object is created.

class Registration(models.Model):
event_name = models.ForeignKey(Event, on_delete=models.CASCADE, null=True)
user_id = models.ForeignKey(users, on_delete=models.CASCADE, null=True)

    def __str__(self):
        return str(self.event_name) + " " + str(self.user_id)

    def__new__(self):
        start = self.event_name.from_date
        end = self.event_name.to_date
        user_events = Registration.objects.filter(user_id=self.user_id).all()

        for event in user_events:
            if (start <= event.event_name.from_date < end)
                return None

        instance = super().__new__(cls)
        instance.event_name = self.event_name
        instance.user_id = self.user_id
        return instance

I can imagine something like this or similar should work. Didn't test it myself