You're missing the point. He did a bad thing. Yes. But punishing him for doing it doesn't undo the bad thing. Right now we have the chance to praise the good thing: switching sides.
What will get more blue votes: penalizing red votes after they're already cast, or extolling the virtues of voting blue, especially using cases of those who have seen the value of switching sides?
You're not wrong that his vote may have caused harm, but he has just as much right to cast it as you have to shun him for it. The real story here is that he learned and is now on a more productive side. We should be celebrating the future, not dwelling on past mistakes.
Ask them to do so themselves, voluntarily.
If they refuse, the argument stands.
If they accept, you won't convince this person.