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You must be insufferable to be around. Here's a novel idea, how about instead of directly resorting to "Your code sucks." we instead say something like "I think there's still some work to do here. Here's the way I would have done this." It's really not that hard to give constructive criticism.
In practice it works like this: "Your code invokes undefined behavior and may fail in the following ways. These corner cases are handled incorrectly. There are a few other cases I'm not sure about, because I have trouble following the logic." "The code is fine. I have tested it!" "It happens to work on you current machine. That's the nature of undefined behavior. It might fail in the future without warning." "I teste…
In your scenario, the person saying this is being the bad contributor, and should be told to explain what they disagree with in the review. If anyone shuts down in a code review like that more than once, they should be finding a new place to work. And that really goes for both sides to be fair -- if I put in effort into a PR I deserve a good constructive review. If it's a public project and this stranger refuses to follow the rules, no harm done in closing the request and telling them to come back later when they're ready to discuss things.
Or you know, however you discipline them. I think we're in agreement that there's no need to resort to personal attacks and frankly someone telling me my code is crap is probably better than being told I should have been aborted in a code review.