And how do you propose filtering them out? There's a reason why college students are using LLMs, they're getting better grades for less effort. I don't assume you're proposing selecting students with worse grades on purpose?
I think what the junior did is a reason to fire them (then you can try again with better selection practices). Not because they use code from LLMs, but that they don't even try to understand what it is doing. This says a lot about their attitude to programming.