Set a maximum quantity for cart items
You should be able to handle this with some logic in a plugin listening to events like:

https://craftcommerce.com/docs/events-reference#commerce_cart.onbeforeaddtocart

The parameters for this include both the order as it stands, and the lineItem being added to the cart, which gives you the opportunity to manipulate both.  You could clear the order, and chec the qty of the lineitem being added is 1 at this point (and change it if needed).

Obviously it would be good to have some front end stuff in place limiting the quantity to 1 as well.