I would expect such a process to be wildly difficult to enforce. On the coasts, smuggling would become a massive business, perhaps funding the activities of massive pirate fleets which would become impossible to remove. Westerners will also begin to run their own smuggling operations, perhaps even going so far as to militarily capture an island or something in order to trade with China.
But the biggest problem with the ban would be Mongol tribes. Some measure of trade was needed with the Mongols (generally a horses-for-manufactured goods trade), and with drastically lower levels of trade the Mongols would begin harassing the Ming Empire in even greater force than they did historically.