The only conceivable defense that exists against this city is the Counter Force/Grand Servants, if the World feels like it's being sufficiently threatened. If worse comes to worse, King Hassan himself would come to clean the mess, but this requires like a catastrophe of epic proportions to happen, which isn't the case here.

So, to be brief, there's nothing anyone in the world can do. It's like trying to poke Academy City from the Raildex franchise. It won't end well.

And that's not even counting the massive economical and political power amassed in Fusang, which is likely enough to sway the decision of any government in the world. This is serious a case of "Deal with it" since you can't do anything else after the process began.