So we are doing champion runs. At the very end of the level, and my friend gets sucked into a stack of candles. Can’t move, can’t jump, can’t do anything. So to fix it, he left, so the bot would teleport to us, as he had the grimoire, and we needed him to get a full run. Before he could rejoin, a level NINE Bardin joined, but we couldn’t kick him because it was the end of the game. So we very politely asked him to leave so our friend could rejoin, and explained the situation. He jumped off a ledge seven times, and proceeded to shoot us with all of his ammo, then run around the wrong way until he died and lost the grim.
TLDR; You should be able to kick someone at any point in the game.