Of course it is possible. Drivers have full control over it once they get in the zone and they are in the required conditions to use it.
They can close it manually when they find yellow flags in the middle of the DRS zone before it gets disabled by race control. Let´s not forget they needed to close it manually in practice and qualifying 2012 in the way into the corners without braking zone (like 130R).
Well, according to the tech spec DRS should be deactivated by either the release of the button or by touching the brakes. There migh be a possibility that the system stays enabled, but I am under the impression that the DRS is "loaded" by the race control signal, therefore "fired" by the driver. But of course there is a possibility that there is a continous signal throughout the DRS zone.
In either case, Alonso activated his DRS seconds before Nico, who was ahead, so either Nico was very slow on the button or Alonso's DRS was working independently from any race control signals.
Edited by ardbeg, 01 August 2013 - 16:00.