You avoid or ignore him. You deal with his deputies, you work with them behind the public eye. Duterte simply wants to be left alone to do his job. If he is not in violation of the UN charter you have no legal or moral authority over him.
But the Philippines will pay a price for this very public snub. I wonder what that would be. Perhaps just the international diplomacy equivalent of our old, "Youths with long hair would be entertained last"
I wonder how many Filipinos especially those living overseas are embarrassed.
Update: 8:45 am
Of course this was a snub. You had to be trying very hard to read this any other way, but this is also sad because is shows pettiness and weakness on the Chinese side. Whoever did that is probably going utto be severely punished by away from the public eye by one of Xi's lieutenants. The Chinese president doesn't need to get involved here. So simple they know what to do.
Again, what if it had been President Trump? What would he be tweeting?
Update: 2:45 pm
Not that slow in expressing regret which he never did when he got personal with Pope Francis. Jack be quick or the Americans would make you pay eh? They are not as kind or patient as God.