Completely understand the frustration here. Mistakes happen, even competent people sincerely trying to do a good job can overlook things, etc. But if it's a pattern of just copying and pasting code without really even trying to understand what it does, that's a big problem that needs to be addressed. And frankly they should feel embarrassed if it happens more than once or twice.

OTOH, delivering criticism in a way that winds up productive for all involved is difficult at best, and the outcome depends on the junior as much as it does the senior. What good is being right if it ultimately just alienates you from your team? Tough situation for sure, and one of the many reasons it's so important to hire carefully (which is itself a whole huge can of worms too!).

Can you simply ask them to walk through their submission line by line with you, explaining what it's doing? If you've never asked that before it might come across as a strange request, but if you phrase it well it's possible this causes them to notice their poor understanding without you ever seeming to point it out.