How do you disconnect a function?

So I'm having troubles with some function

local function testF(Data)
	Mouse.Button1Down:Connect(function()
		print(Data)
	end)
end

testF("TestString")

--LATER ON

testF("NewTestString")

before this function is fired, clicking the mouse would NOT cause the output to print TestString

but once it is fired, clicking would then print TestString, how would I go about this so that this function is cancelled so that it would cease to print TestString, the function still believes the data is TestString, instead of NewTestStringdespite firing the function once again.