Probably by not letting them check in code they don't understand. This creates a lot of overhead, but it turns out you simply can't skip that time investment. Somewhere along the junior->senior developer (in terms of skill, not time) is a big time cost of just sitting down and learning. You can pretend to skip it with LLMs, but you're just delaying having to understand it.

So the benefit of LLMs is negligible in the long term, for beginner/junior programmers, as they essentially collect knowledge debt. Don't let your juniors use LLMs, or if you do, make sure they can explain every little detail about the code they have written. You don't have to be annoying about it - ask socratically.