[Gandalf Parker]

I can see the point. Some large games can easily take a month to set up. And might clearly have stated at the start that it was meant to be competitive for bragging rights (or some other wording, king of the hill, ladder, gunslinger). It might be a serious test of strategy game being ruined in the eyes of the other players by a non-serious player. Or the opposite, a meant-for-fun game being ruined by too serious a player. If its clear fairly early in the game that a player is not one that should have been included then removing them would become an obvious topic just in fairness to the other players who have waited so long for the game to start. Why punish the whole game because someone managed to slip in?

My stance would be that The person who created the game has the right to allow anyone into their game. And as an extension the right to revoke that at any time for any reason.
There are nice ways of doing it which I think has mostly been done here. But even done badly Id still tend toward his-game/his-rules.