You would have to disconnect the Thunderbolt cable from your MacBook Air. The TBD does not have a power button and only works as a display when connected to a computer. I don't know of any setting to do this.
Bit of a revival, but I've not got a Thunderbolt display (yet, waiting for the updates to get one to accompany my Mac mini) but a simple: Ctrl-Shift-Eject on my iMac puts my display to sleep, can't see why it won't work on a TBD? Like to know if it does?
Yeah. How genius. You got an handicap display and found a way to make it less handicap. I unfortunately can't fix it because my keyboard do not have a "Eject" button. Again, brilliant. They remove the Delete key and put in an Eject. They really want to ejectulate.