It was not the job of "anyone" to stop it. It was the job of the New Orleans police, led by the mayor. Riots are easily scotched in their earlier stages, but very difficult to stop when they have been permitted to get out of hand.
WHY WASN'T MAYOR NAGIN IN NEW ORLEANS FOR THIS CRISIS? You didn't catch Rudi Giuliani abandoning New York in the crunch. Why wasn't he there giving orders to the police?
If the New Orleans police were incompetent to keep order, why didn't Mayor Nagin put someone in charge of them earlier who could have straightened things out?
Because, as people have pointed out on other threads, he had no interest in governing the city, but only in having a good time collecting graft, as politicians in Louisiana have traditionally done. That's always easier if law enforcement is lax or nonexistent.
But when the crisis comes, these guys bug out--and blame others for their own deficiencies.
Political appointees at every level from top to bottom in corrupt La.