How to use debounce in this script?

function Shutdown()
	local m = Instance.new ("Message")
	m.Parent = game.Workspace
	m.Text = "The owner is shutting down the server..."
	wait(3)
	game.Players:ClearAllChildren()
m:Destroy()
end

script.Parent.MouseButton1Click:connect(Shutdown)

This is a script in when you click the shutdown button, everyone gets banned from the game.

How do I add a debounce to prevent multiple messages coming in, and how do I make it so ALL the messages get deleted? (Because whenever the button is spammed, it makes multiple messages that don't go away.)

You want to have some variable marking whether or not you're allowed to show the message; call it canShutdown for example.

It needs to start so that you can press the button once and it will work:

local canShutdown = true

You want the messages to only appear when you're allowed to shut down:

local canShutdown = true

function Shutdown()
	if canShutdown then
		local m = Instance.new ("Message", workspace)
		-- Using second parameter of Instance.new makes it a little neater!
		m.Text = "The owner is shutting down the server..."
		wait(3)
		game.Players:ClearAllChildren()
		m:Destroy()
	end
end

script.Parent.MouseButton1Click:connect(Shutdown)

And you want to make it so that you can't shut down while there is a message already:

local canShutdown = true

function Shutdown()
	if canShutdown then
		canShutdown = false -- No longer can make new ones
	
		local m = Instance.new ("Message", workspace)
		m.Text = "The owner is shutting down the server..."
		wait(3)
		game.Players:ClearAllChildren()
		m:Destroy()
		
		canShutdown = true
		-- Now we can press it again, since old message is gone
	end
end

script.Parent.MouseButton1Click:connect(Shutdown)