It’s the obvious solution, isn’t it? Wait for him to arrive, and then jail him. Along with as much of the Iranian UN delegation as we can catch.
It’s not like the Iranians haven’t already established the standard that diplomatic immunity doesn’t exist between the US and Iran.
We can release them the day a new Iranian government more to our liking is in power; after all, the Iranians apparently released our people when they figured out that Reagan wasn’t going to roll over like Carter did.