If trolling truly starts to become a threat, there is legal recourse. It's just as illegal to threaten someone with murder on the internet as it is on a street corner. As for sending pornographic images, that's already against the Twitter terms of service, so all one would need to do is report the troll for spreading porn, then wait for Twitter to remove them from the platform. There's no need for a government account to block people.
"It's just as illegal to threaten someone with murder on the internet as it is on a street corner" -- this interests me. On a street corner the face-to-face threat is far more real and the threatened outcome more likely than on the internet where the lack of physical presence leads people to rant and vent in offensive ways that don't amount to much. Half the time the people engaged are on different continents with made up names, so even if someone got sufficiently worked up to pursue another s/he wouldn't get very far unless he was so psychologically divergent that frankly s/he's going to kill someone else soon enough that twitter policing is irrelevant. I think twitter, and possibly the net, should possibly be at least partially treated like MMA - sport, not taken too seriously, get in, take a beating, give a beating. I haven't give this much thought, but basically there seems to be no acknowledgment of the fact that the virtual world is not physical, and it seems to me that we interact with the two differently and that should be considered.
Even in person, the laws distinguish between "true threats" and hyperbole, which will take both the content of a threat and its context into account.