Most restrictions on classified information are enforceable because those with access to the information have voluntarily surrended their first amendment rights in respect of classified information as part of the agreement they entered into when they obtained a security clearance.
It's much less clear when you are talking about someone who hasn't agreed to the clearance process. In 1979 the Department of Energy tried to prevent The Progressive magazine from publishing an article by Howard Morland describing the design of the Hydrogen Bomb. The article was derived from unclassified sources, but the government argued that information about atomic weapon design is "born classified". There was no conclusive judgement, as the government eventually dropped the case and the article was published.
It's much less clear when you are talking about someone who hasn't agreed to the clearance process. In 1979 the Department of Energy tried to prevent The Progressive magazine from publishing an article by Howard Morland describing the design of the Hydrogen Bomb. The article was derived from unclassified sources, but the government argued that information about atomic weapon design is "born classified". There was no conclusive judgement, as the government eventually dropped the case and the article was published.