I've been asking all my associates about what they think the worst problem in the world was (so that I could find a solution of any credibility whatsoever.) All of them said that violence was the worst problem.
It's not an easy problem to solve. Every nation on earth has some degree of violent crime. Several wars are raging as I type this sentence. A few other wars are threatening to happen. Some violence is unreported, like people in bars getting into alcohol-fueled fistfights, or children being spanked. (Weather or not spanking is legitimate as a discipline method is another story. My point being, it is unquestionably violent.)
Aside from a world-wide prescription to vallium, I think the best way to reduce violence is a strong and honest police force. Police forces take issue to assaults on citizens under their protection. They have the brute force to do something about it, and in most communities, the legitimacy to make pariahs of any opposition.
Wars can be reduced with extra diplomacy, but not entirely eliminated. If two nations have overlapping land claims, either one must relinquish the claim or there will be war. Nations have also fought wars over issues of prestige and power. Nothing makes a nation seem more powerful than to humiliate a smaller nation. Thankfully, this tactic is largely going away due to changing politics and larger, more it-would-be-suicidal-to-challenge military forces.
I do not know how to prevent bar violence, other than immediately tranquilizing drunks that so much as look angrily at each other. Intra-family violence is also tricky because everyone involved often feels that it is justified. Some parents do feel that children must be spanked when they misbehave, or they will never learn to be reasonable people. Some people feel that a fistfight is the only way to solve severe enough interpersonal problems. I do not have an answer.