You would have to delete everything inside the intro.lvl, make the game screen as small as possible and put a lava wall at the very left of the screen that is one block large.
Do NOT delete the intro.lvl itself or else you won't be able to use the ''Play Game'' button
EDIT: Actually, putting a lava wall will make the game lag even more. Just trap the players in a room using less blocks possible.
Since they can't leave the section, they can't make more players appear thus stopping the cycle
Some project I'm working onto.