And that body would need firearms.
Firearms aren't the problem. It's states because states use firearms to get resources with which to conduct war. But states are unavoidable because without states we'd have caudillos -- warlords, heads of tiny states that will in due time coalesce into large states, and we're back at square one.
A single world state wouldn't be a solution for the same reason that larger states have at times fallen apart into smaller states: there will be resentments and tyranny. Indeed, a single world state would have to be tyrannical just in order to stay together, and its government would be highly non-representative for natural reasons, namely that having it be highly-representative would mean it having -impractically- hundreds of thousands of representatives.
You're not the first to think that extreme violence could bring peace, but it's an illusion. Huge majorities of humans must know internal peace and have tremendous wisdom for us to enjoy peace on Earth.
And requiring global controls on 3D printers, CNC machines, any pressure bearing metal, and a range of chemistry and fertilizers.
All this after a door to door search for weapons and ammunition. What army exactly is going to be kicking down my door to confiscate these at gunpoint?
I’d say in an ideal future humans wouldn’t want to create weapons.
But in an ideal future…?
But the system isn't perfect.
Neither will anything else. Ultimately it's your life and you're the only one that's going to have to defend yourself at a random time. If a crime is committed against you then you're just a statistic for the government. Nothing more.
Small scale 'war' should also be avoided. Guns make personal conflicts much more deadly.
Tyrants hate that.
Before firearms the world was mostly ruled by "nobles" (i.e., those who had extorted enough from the peasants to afford weapons and armor, and had the free time for the required years of training).
Now, it isn't. That didn't happen because the "nobles" suddenly decided to turn into nice guys.
Oh, buddy, do I have some news for you
the genie left that bottle more than a hundred years ago
But not bombs, knives, chemicals or sources of radiation?
Well.. good luck.
A few years of having the greatest power on earth turns 99% of otherwise good men into non-humans. That’s just the fact of life, explained thousands of times throughout history. That’s why you have to elect a new guy every few years, cause the old one starts to lose it. Try removing him if his gang is the only gang with firearms.
A body like interpol would investigate the entire world, to surveil if anybody is trying to make firearms.
People would make bows and spears, but it would be more difficult to conduct war.
It would not make war and violence impossible, but it would certainly be a better world.
EDIT: lots of pedants:
obviously there would be an joint army answering to an international body whose only mission is to seize and destroy weapons.
firearms includes bombs and other lethal ammunition, of course. Not knives.