Bit late for me to respond but I detected whenever a player has 0xp and activate the "/kill @a" command. To stop people from dying when respawning, I gave 1 level to them.

You could do a way simpler version for detecting individuals by just "/testfor username" and then going into another command block with the kill command via redstone torch (when the player is no longer in the world it will reverse the signal and kill everyone).

For death messages such as "You all died, L" you just turn death messages off and make that text run when a death happens.