Trying to break a Heartbeat help?

The :connect method returns a RBXScriptSignal.

That signal has a :disconnect() method which stops it from listening. Thus your code would look something like this:

	local loop = RunService.Heartbeat:connect(function(Step)
		if keeploop then
			script.Parent.MaterialPicker.Toolbar.Loader.Rotation = script.Parent.MaterialPicker.Toolbar.Loader.Rotation + 5
		else
			script.Parent.MaterialPicker.Toolbar.Loader.Rotation = 0
			loop:disconnect()
		end
	end)

I have some more recommendations about improving this code.

Don't use coroutine.wrap when spawn is sufficient -- it's so much shorter, cleaner, and shows your intent better:

	spawn(function()
		wait(timspin)
		keeploop = false
		script.Parent.MaterialPicker.Toolbar.Loader.Visible = false
	end)

even better, use delay instead of spawning something that waits:

	delay(timespin, function()
		keeploop = false
		script.Parent.MaterialPicker.Toolbar.Loader.Visible = false
	end)

instead of setting keeploop, you might as well just :disconnect the connection. You can also eliminate timeforspin since just the parameter works fine:

function mainmod:Spin(timeforspin)
	script.Parent.MaterialPicker.Toolbar.Loader.Visible = true
	local loop = RunService.Heartbeat:connect(function(Step)
		script.Parent.MaterialPicker.Toolbar.Loader.Rotation = script.Parent.MaterialPicker.Toolbar.Loader.Rotation + 5
	end)
	delay(timeforspin, function()
		loop:disconnect()
		script.Parent.MaterialPicker.Toolbar.Loader.Rotation = 0
		script.Parent.MaterialPicker.Toolbar.Loader.Visible = false
	end)
end

Since you say it 5 times, you should just save script.Parent.MaterialPicker.ToolBar.Loader to a variable:

local gui = script.Parent.MaterialPicker.ToolBar.Loader

function mainmod:Spin(timeforspin)
	gui.Visible = true
	local loop = RunService.Heartbeat:connect(function(Step)
		gui.Rotation = gui.Rotation + 5
	end)
	delay(timeforspin, function()
		loop:disconnect()
		gui.Rotation = 0
		gui.Visible = false
	end)
end