Once information has appeared in a public place anywhere, it's almost impossible to prevent it from being available ANYWHERE.
Sure, there are cases where the information seems so un-interesting that nobody will bother to copy it before the state manages to seize all copies of it. There are also cases where loyalty to the state (or employer, or church, or fraternity) is so strong that thousands of trusted people may have copies but they won't distribute them and you (the state/employer/church/fraternity officials) know it.
There are also cases where fear of even possessing the information (plus the fact that most people simply wouldn't want to possess it) means the state has a much easier time keeping track of those who are both un-afraid and who might actually want to possess it (classic example from country that generally values free speech but makes a few exceptions: child porn).
Other that these and a few other edge cases, once something is published it's pointless for a country that claims to value free speech to try to declare it a "secret after the fact." Unless of course the point is disabuse your citizens and the world of the idea that you (the state) value free speech, in which case go right ahead, you'll soon achieve your goal.