How can I make a leaderstat or leaderboard take away health if it is low?

So I'm making a RPG game, and I have Gold, Level, and XP, but I added my own other leaderstat called "Ki" and what it does is whenever you use a energy weapon like a "Ki Blast" it would lower your ki. But whenever It shoots it lowers the Ki and Ki regens but if you keep clicking the ki goes into negative numbers and so you can shoot infinitely. I need to make it so it stops at 0 or if you go into negative numbers it subtracts health.

Here is the RPG Script:

function onXPChanged(player, XP, level, c)
	if XP.Value>=level.Value * 100 then
		XP.Value = XP.Value - level.Value * 50
		level.Value = level.Value + 1
	end
end

function onLevelUp(player, XP, level)
	if player.Character~=nil then
		for i = 1,5 do
			local fireworks = Instance.new("Part")
			fireworks.Shape = 0
			fireworks.formFactor = "Symmetric"
			fireworks.Size = Vector3.new(1,1,1)
			fireworks.BrickColor = BrickColor.Random()
			fireworks.CFrame = player.Character.Head.CFrame + Vector3.new(0,2,0)
			fireworks.Parent = game.Workspace
			game:GetService("Debris"):AddItem(fireworks, 2)
			fireworks.Velocity = Vector3.new(math.random(-30,30),math.random(-30,30),math.random(-30,30))
		end
	end
	local m = Instance.new("Hint")
	m.Parent = game.Workspace
	m.Text = player.Name .. " has powered up!"
	wait(3)
	m.Parent = nil
end

function onLevelUp(player, XP, level)
	wait(0.1)
	player.Character.Humanoid.Health = player.leaderstats.PwrLvl.Value * 100 + 100
	player.Character.Humanoid.MaxHealth = player.leaderstats.PwrLvl.Value * 100 + 100
	
--[[
	local stuff = player.Backpack:GetChildren()
	wait(5)
	for i = 1,#stuff do
		local name = stuff[i].Name
		if game.Starterpack:findFirstChild(name)==nil then
			stuff[i]:Clone().Parent = player.Backpack
		end
	end
--]]
end

function onPlayerEntered(newPlayer)
	local stats = Instance.new("IntValue")
	stats.Name = "leaderstats"

	local xp = Instance.new("IntValue")
	xp.Name = "XP"
	xp.Value = 0
	
	local level = Instance.new("IntValue")
	level.Name = "PwrLvl"
	level.Value = 1

	local g = Instance.new("IntValue")
	g.Name = "Zeni"
	g.Value = 10
	
	local c = Instance.new("IntValue")
	c.Name = "Ki"
	c.Value = 100

	c.Parent = stats

	stats.Parent = newPlayer 

	xp.Parent = stats
	level.Parent = stats
	c.Parent = stats
	g.Parent = stats

	stats.Parent = newPlayer

	xp.Changed:connect(function() onXPChanged(newPlayer, xp, level) end)
	level.Changed:connect(function() onLevelUp(newPlayer, xp, level) end)
	
	newPlayer.Changed:connect(function (property)
		if (property == "Character") then
			onPlayerRespawned(newPlayer)
		end
	end)
end

game.Players.ChildAdded:connect(onPlayerEntered)

while true do
players= game.Players:GetChildren() 
for i = 1, #players do 
if players[i].leaderstats.Ki.Value ~= 100 then
players[i].leaderstats.Ki.Value = players[i].leaderstats.Ki.Value + 1
end 
end 
wait(0.3) 
end

--My original attack script and your version. ( I couldn't get yours to work)

function onButton1Down(mouse)
game.Players.LocalPlayer.leaderstats.Ki.Value = game.Players.LocalPlayer.leaderstats.Ki.Value - 20
end


function onS(mouse)
mouse.Button1Down:connect(function() onButton1Down(mouse) end)
end
script.Parent.Selected:connect(onS)

Yours

script.Parent.Selected:connect(onS)

game.Players.LocalPlayer.leaderstats.Ki.Value = Ki

function onButton1Down(mouse)
    local KiToRemove = 5
    if Ki >= KiToRemove then
        Ki = Ki - KiToRemove
        function onS(mouse)
	mouse.Button1Down:connect(function() onButton1Down(mouse) end)
end
	script.Parent.Selected:connect(onS)
    end
end

You have not given me any of the code that actually removes Ki, but this how you do it:

function Attack1()
	local KiToRemove = 5
	if Ki >= KiToRemove then
		Ki = Ki - KiToRemove
		-- Attack code
	end
end