How would I change the default shutdown message?

Judging from the body of your script, I see it as an admin kicking a player from the game, rather than the actual game shutting down.

It wasn't too long ago, but ROBLOX updated the :Kick() method of the player instance to take in a string argument, the message you want displayed in the red box whenever a player is disconnected.

game.Players[Player.Name]:Kick("An Admin has removed you from the Game.");

ROBLOX does not have a feature to change the shutdown message. Although, ROBLOX has a Kick() method to players. I use it in the console in my games if I ever need to kick someone.

Here is the basic usage:

If you want to kick a player in a server, you can use the :Kick() function through the player. The only argument that can be associated with the :Kick() function is the string: kickMessage argument. Below is some examples and what it does:

game.Players.PLAYER:Kick() -- This will kick the player with the default message

game.Players.PLAYER:Kick("You were just kicked. Why? Because you were trolled")

You may already know the message will appear in a red box at the top of the screen.

If you need any more help with this, see the wiki page about it here.