I don't think the problem is the LLM.

I think of LLMs like clay, or paint, or any other medium where you need people who know what they're doing to drive them.

Also, I might humbly suggest you invest some time in the junior dev and ask yourself why they keep on producing "garbage code". They're junior, they aren't likely to know the difference between good and bad. Teach them. (Maybe you already are, I'm just taking a wild guess)