No Stupid Questions

Equal distribution of wealth and power, along with trying to create universal standards of communication (I.E. speaking reading and writing) would help probably.

Plus universal access to education and birth control. And ENFORCING equality by making sure there is no such thing as inherited wealth. Each new generation would have to start form the same point. I know it might seem a little extreme, but I think it's the only way to prevent oligarchies, monarchies, and huge concentrations of power that distort society from forming.

Plus, guaranteed clean food, water, housing, and basic healthcare for all. Get rid of the reasons for conflict, and make sure those who want to start conflicts can't ever mass enough power to start wars and we won't have them. A lot harder to justify fighting when you personally have to do it all yourself because you can't order others to just do it for you.

Put the ones in charge to the front line.

Like others said, struggle is in human nature. But it is possible to shift it to other domains: art, science, exploration. To prevent this stuggle spill back into physical violence, there should be broad consensus on basic rules, effectively enforced.

So I'd say, build this consensus, which will probably need to rely on abundant renewable enengy, some form of UBI, equality, and stronger international institutions, but will not spotaneously evolve towards unsupportable preferences of some groups.

Simple. We make war illegal.

Nuclear deterrent. If that fails, nuclear annihilation. Either way, there will be no more wars.

Kill every human, or all but one. As long as there’s at least two people, there will be conflict.

Create an Artificial General Intelligence with the sole purpose of satisfying people's values through friendship and ponies.

It would take way more but the first step it would be to remove religions. And not only because most are not peaceful in principle but also because some religions see others as a target. Even small difference in beliefs could spark a vicious and long standing conflic, like Catholics Vs protestants.

An unfortunate combination of somehow achieving a post-scarcity, non-capitalist society with robust resource and population control.

But people are greedy and want "freedoms" that run counter to this.

The only way to end all wars is for all people to fully control the instinct of domination with rational mind. It's all about education and self-awareness. Making people rational creatures

Colonizing space

Bonobo way

Humans are ultimately animals. We strive for dominance on an individual and societal level. We can run societies in a way that helps to minimize violent conflict but I don't think you can ever eliminate it. Not while still being what we know as human.

I think the solution is to have enough renewable energy for all of humanity.

You'll have to find a way to eliminate greed

Edit: or no people so you have nobody to start wars

Impossible. There is 1-5% of the population who are born anti-social. It's very difficult to socialise them, and some never do. They are vastly overrepresented in prisons. These people sometimes get into positions of power. Particularly in societies which value authoritarianism, like Russia and China. This cannot be helped. Instead, we must mitigate the effects of their reach. NATO, for example, is an excellent way to stymy these dictators.

Get rid of religion

Democracy and the rule of law. Sounds banal but it's the best shot we have. People crave power. People who have power want more of it, which is the underlying cause of a lot of wars. People who have power will do absolutely everything to keep it, including starting or prolonging wars.

Therefore people who want power - regardless of the system of government - are inherently unsuited to use it responsibly. The solution? Give power only on a strictly time limited basis and within a framework of transparency and control. If you misuse the power lent to you, you will be held responsible.

This won't prevent shitty governments from happening, but at least it provides a way to get rid of said shitty governments. So, far from a perfect solution but the only realistic one.

Make tea

Send everyone in charge to an Enya concert. "We can sail, we can sail, With the Orinoco flow..."

I think it would be bad to end all wars.
War sucks, no doubt. But think about this: throughout history, fighting has led to larger, organized societies that decrease violent deaths. These societies also improve living standards and boost our wealth. So, war makes us safer and richer.

WashingtonPost - In the long run, wars make us safer and richer (April 25, 2014)