Minecraft wiki entry for Command_blocks_and_functions, associated text: Create a hostile mob (so it disappears when switching to Peaceful) that never despawns if the player is far away. To do so, use a name tag on the mob (or third party software if you are playing in a version previous to 1.5.2) and place it on a pressure plate. Make sure it will keep it pressed infinitely, so make sure surround it with 2 high blocks or fences, so that it can't jump or walk away. Make a redstone circuit that causes a command block to activate when input from pressure plate ceases to exist. Place a message inside the block that informs the player on chat that the map has been broken and must be redownloaded. This way, switching to Peaceful will cause the mob to vanish and activate the message. You can even make a teleportation command block so that a cheater is banished from the map to a small room without a way out as soon as they switch to Peaceful.