Nothing, because it wont happen, because it's just a stupid thing that happened a few time that's blown out of proportion by the news. Kinda like the Bathsalt zombie apocolypse.
If I did by some direct action of god himself come across this situation, I'd put a bullet in the idiots chest and then make sure he fully understood that he was going to die in a f**king clown suit. I mean really, aside from torture, I cant imagine a worse way to die than in child molester makeup and a wig.
And if I don't have a gun, well then I can use the pocket knife I always carry. It's not like someone in a clown suit will actually be able to fight back.
And yes, stuff like this stupid clown craze does urk me. Smarten up, cattle.