If I understand this right, you want to be able to remove players from games in progress?
What's to stop somebody from removing a player because they are losing/winning/an obstruction?
AFAIK Diplos are games with rules that are already made up, not part of the warlight infrastructure. Why would Fizzer need to make something to support that specifically, when you could just choose who you play with? Or remove people before the game starts, who are not active, or not joining?
No, talking about removing a person from a game while looking for players. The invite phase. In case someone joins and proceeds to be a dick.
And this is also valid for FFAs, not only diplomacies. FFAs are a part of the "warlight infrastructure", so...