This seems fairly neccesary at this point.  There are many times where I've seen people in #demigodgame on IRC screaming at so-and-so to stop trying to join their game.  It's borderline griefing.  The OP is most likely correct that there will be some people who will simply keep trying to join a game out of spite after they've been kicked, thus increasing the chances of the lobby freezing or crashing.

NOoooooooooo.  I am one who will keep joining a game when kicked... But it is because sometimes, like many others, I get a ping spike when joining.  It will be anywhere from 400-1k for at most 3-4 second, usually less.  After that it goes down.  And I dont havy any issues in game, up to and including 4v4. 

I agree though that a ban is needed, it seems silly for it not to be there.  I only hope that some day, hosts will stop being so ignorant.  It seems half of them dont understand what ping means or how it works, let alone anthing else to do with the connection system.

This is for custom game, not Panth or Skirmish... Now if only i could play at least 3v3 in those without any AI players....

Banning in customs is nessisary for a number of reasons.  Mainly that its the hosts game.

Yes I agree that its quite irritating if a person who lives in Japan joins a 3v3 EU game and gets 400+ ping. I've experienced multiple games like this but you can always ask the person to leave and most of the time they directly leave or do so within 5 minutes.

Even if its 'only' a custom game no permanent ban should exist in a game, a temporary ban is pushing it too. IMO the kick button is fine as it is. Just warn the person that he has a high ping and that he needs to go.