Hmmm, that's a good question. We never have anybody in the backseat of our car so it would be good for us to close them off if you can get more airflow in the front. I have a hard time feeling the air hit my face with the two vents that I can point at me. I know the front vents can be closed off, so I would guess that the rear vents have that same little dial so you can close them off?
If you don't have tri-zone then you probably can't turn it off like I said. In CUE it would be a button labeled Rear which would bring up the rear controls. In every dual zone car I've had the rear is controlled by the passenger side, but you can usually at least turn off the fan for the back.