You won't be able to buy anything if you decide to let your Pro membership lapse - everything in the Store, including pizzas, is for Pros with the exception of one SBR tshirt and hat. But your points won't disappear - they'll just be worthless since you can't cash them in for anything. And if you decide to become a Pro again later down the road after letting your membership expire, you'll only be able to carry 5k back over.