Only the game-master of a multi-player-game can remove players and only if the game has not started yet.
 
A user may notify the game-master that he wants to be removed from the multi-player-game, or the game-master no longer wants the participation of an already joined player (for whatever reason). In the latter case politeness dictates, that the game-master informs the to-be-removed player about this decision.
 
To remove a joined player open the game-players page for the multi-player-game and in the players list click on the "Trashcan"-icon () in the Actions-column for the joined player you want to remove. You will be asked for confirmation of the removal.