Just discovered a new hidden feature:
If you slide either of the driver's side A/C vents ALL the way to the left, then a secondary flap comes down (it's not very visible, as it's behind the vent's directional grills) and closes the vent completely. Ditto for the passenger side, but you have to slide them all the way to the right on that side, instead of to the left.
You can also confirm/test this by turning the fan way up, then listening to how the fan noise changes suddenly (and the other vents blow harder suddenly) when you hit the "full stop" close position.