China declared all cryptocurrency transactions illegal Friday, trying to shut down its citizens’ use of digital currencies that operate free of government control. It was just the latest in a spate of restrictions President Xi Jinping’s Chinese Communist Party is imposing. That the isolationist measures are becoming more drastic has a silver lining: They’re a sign of how increasingly difficult and elusive such government control is in a globalized economy and social media age. Last week, the Chinese equivalent of TikTok, a short-form video app called Douyin with 490 million users, announced a 40 minutes-a-day restriction for its users under…