How do I Disable certain movement keys?

hey bud, ContextActionService is what you're looking for. but here you go

local CAS = game:GetService("ContextActionService")
local FREEZE_COMMAND  = "Stunned"

CAS:BindActionAtPriority(
	FREEZE_COMMAND,
	function() 
		return Enum.ContextActionResult.Sink
	end,
	false,
	Enum.ContextActionPriority.High.Value,
	Enum.KeyCode.W,
	Enum.KeyCode.S
	)

--CAS:UnbindAction(FREEZE_COMMAND)
--this is just if you ever want to re-enable movement in the Z axis

just an important tip, you're gonna want to restrict the camera's movement too, but i can't really help with that