I'm right now planning a story out and I need to figure what products were banned or restricted that held back technological development or growth. For example, in the 1920s, the US banned all export of helium, thus limiting the availability of helium for Germany to use in its Zeppelins, making them rather dangerous. In turn, the US limited its airship development by producing USS Shenandoah and USS Los Angeles and then not producing much more after that. What was banned like that?