Root ginger freezes really well. If you buy a large chunk of root ginger and know you won’t use it before it goes off – freeze it.
Keep it in the freezer until you need it. It keeps for about six weeks. You don’t need to thaw it to use it. Use a sharp knife or a mandolin (careful!) to shave off what you need. (Don’t worry about peeling the skin – you can’t tell it’s there when you chip it from the frozen block.)