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I think you're denying him an important chance to do error correction via that decision. (This is a particularly important concept in CR/FI)
curi evidently wanted to change some things about his behaviour, otherwise he wouldn't have updated his commenting policy. How do you know he wouldn't have updated it more if you'd warned him? That's exactly the type of criticism we (CR/FI) think is useful.
That sort of update is exactly the type of thing that would be reasonable to expect next time he came back (considering that he was away for 2 weeks when the ban was announced). He didn't want to be banned, and he didn't want to have shitty discussions, either. (I don't know those things for certain, but I have high confidence.)
What probability would you assign to him continuing just as before if you said something like "If you keep continuing what you're doing, I will ban you. It's for these reasons." Ideally, you could add "Here they are in the rules/faq/whatever".
Practically, the chance of him changing is lower now because there isn't any point if he's never given any chances. So in some ways you were exactly right to think there's low probability of him changing, it's just that it was due to your actions. Actions which don't need to be permanent, might I add.