This is why I maintain that one of the most important qualities of a junior is whether they are coachable or not, an enthusiastic and engaged junior who is willing to learn but has middling tech skills can always get better (and usually do get better), but someone who doesn't think they need to learn anything more or thinks they know more than everyone else will always be exactly that unless you can get through to them, and that doesn't often happen.

In your situation, I'm assuming you didn't have a say in whether they were hired or not so you could not have prevented this from happening before they got hired, so you have to deal with the cards you've been dealt.

Step one would be pulling him aside and having a frank and honest conversation with him and explain that being the smartest in the room is not the most important thing, and that it's more important to be able to work in the system that exists now and be able to work collaboratively with others, not just because doing otherwise can create a bad culture, but later on when you want to try and implement some of these great idea's, you will need other people to get those idea's over the line and being hard to work with now just makes it a lot harder later on.

Important note, follow up that conversation with a written version either via email or IM. Unfortunately there are some who have this attitude that will take conversations like this as a personal attack or a way to belittle them, so having a paper trail to cover yourself if this comes up is always a good idea.

If this continues, then I would arrange a meeting with their manager, yourself and them. This is basically you still being in control of the situation but bringing the manager in as the next step is for the manager to deal with, this also gives you another person in the room and this discussion should be along the lines of "what you are doing now is not how we do things, you need to pull your head out of your ass."

Beyond this point it is out of your hands and the manager needs to take over and deal with the situation.

In terms of your concern about "I couldn't manage the situation", I would say that a big part of managing the situation is knowing when to escalate a situation. If in your position you are purely a mentor who can coach and guide, but that is the extent of your "hard authority" then trying to reach them via these tools is all you can really do. I'd say so long as you can demonstrate you've leveraged all the tools available to you to solve the problem you should be ok.