Just a little bit

This guy wiped out an entire race with a thought and your plan is to do what exactly?

Me personally? I’m going to back away slowly without giving him my name.
It's like that scene in "The Dark Knight"
Lucius Fox : Let me get this straight, you think that your client, one of the wealthiest and most powerful men in the world, is secretly a vigilante, who spends his nights beating criminals to a pulp with his bare hands, and your plan is to blackmail this person?
[Reese's face falls and Fox smiles]
Lucius Fox : Good luck.
I'm sure Picard could get a hold of Q and make a deal.
- Q would have said,
  "Oh the Husnock? Horrible, simply horrible species. Good riddance. Did you ask me here to give Uxbridge a medal or something? Because he certainly doesn't deserve it! Why I've already genocided three species this morning and didn't even get a thank you."
With a thought, because he was pissed off, and Wesley's big brain idea is to fuck with him. Picard had the wisdom to shrug and go "whatcha gonna do?"
Fiesta!

That guy put himself in solitary confinement, forever. What could the feddies do to him that is more fitting?

Pipe in an endless loop of "Baby Shark" and Justin Bieber?
- Taylor Swift. I've got that "wildest dreams" one stuck in my head. It's an anxiety thing.
Is it really confinement if you can leave of your own volition?
Also, hang him.
- Yes, let's put a noose around the neck of someone who is essentially a god.

I mean, I sort of imagine it to be less the "rule on the books" part, and more the "do we actually have the physical capacity to enforce those rules" end of it. They cant really imprison him (I mean while he's feeling guilty he might stay willingly, but they cant keep him in if he eventually changes his mind, so itd more be him imprisoning himself). Trying to despite the futility of it would seem somewhat dangerous, because again, if he should ever change his mind, you clearly dont want to seem hostile to something with that kind of power, especially when you dont have it. Saying "Our law is not sufficient for you" could just be interpreted as the most diplomatic way given his mental state to justify leaving and not returning.

If the Federation was in the business of putting higher beings on trial, don't you think the second they learned Q was human they'd slap him in a courtroom so fast it'd make his head spin?

Shut up, Wesley! We must be circumspect with those who could visit genocide upon US with a thought. Also, don't bring up how often I challenged Q when he could have done the same or I'll just tell you to shut up again.

COMPUTER TURN OFF THAT NOISE

Genocide is systematic.

This was instantaneous.

Yea, but genocide is like way to much paperwork

"What is your kind called?"

"We are Ferengi."

The dude snapped when he's loved ones were killed, that is considered exculpatory of violent actions in almost all legal systems. The difference is that instead of a knife or a gun he had almost omnipotent powers of destruction.

In an ideal society he would get psychological counseling to deal with the trauma and ensure it doesn't happen again, but I think it's obvious he was a bit above Troi's pay grade.