The Prince finds them fighting in the streets in Act I, scene I. He lectures them that this is the third time that they've bloodied the streets because of their old family rivalry. Not only are they drawing blood with deaths and injury, they're making a mess of his streets. Their noise has also caused problems. He is saying that not everyone shares their disgust with one another, so they need to shape up--or else!
His punishment to them is death if they begin fighting again.
Not only does the continuous fighting show that there's no control, the Prince himself doesn't control the situation correctly. If he had done so correctly, there wouldn't be 4 fights. He also decides to give a 4th warning and avoid actual punishment. For that many times, I doubt that warnings are acceptable anymore. Also, when the prince gives out the warning, he doesn't even bring the two heads of the families together. He takes them each separately. If you're going to solve a feud, bring the two opposing sides together to see what's wrong! Not only this, the Prince comes completely late into the scene to come even close to stopping the feud from ever starting...
He does give the warning that their lives will be taken, but the real thing to look at is that he doesn't actually deal the punishment and it's been 4 straight times.
Main point: The Prince can't control the fighting and has bad timing and doesn't know how to control various situations, showing his lack of autonomy...and his warnings, don't actually help the city and only lead to more fighting which only bloodies the streets even more.
I do realize that this entire answer doesn't completely have to do with the question, but it does give you a compare contrast to the ending when the Prince actually does the correc things and this could give more depth to an essay anyways...