No, they couldn't really ban a movie for something like that. Freedom of speech is still too important to us. They made a bigger deal about The Passion before it was released, but I think that actually helped boost ticket sales by having it constantly being discussed in the media. For a film to get banned in the states it would have to be really extreme. I think the more likely scenario would be the MPAA giving it a NC-17 or X rating then only getting released to cinemas that will show films with those ratings. Quite a few cinemas in America refuse to show anything above the R rating. But again, this would be for content such as extreme violence or sexual situations. Not for children using guns, all they can do is complain about things like that.