Yes, it's not that the player cannot leave, it's just that if they decide to they lose 100 points.
Perhaps each month a player could be given one-two free eject passes so that they can leave if they have to, without losing points.
The key here is to reduce the occurence of someone entering a room, getting a powerup, killing a couple of people, and then leaving before they are killed themselves.
One important thing that needs to be noted, is that if a player were to enter a room, and everyone else leave... he/she would no longer be required to meet the win/losses quota for that room. Also, this would be calculated when the user hits the eject button. So if someone new entered the room after the eject button was pressed, the user would not be required to play the new person.
I think another good idea would be to set a default for the win/loss of each room to 10 and not allow it to be set less. Otherwise, the same problems would still exist.
The snow room has the problem of whichever player gets the first powerup pretty much wins. So, perhaps defaults and minimum values should be set for all rooms to keep everything fair.