you could use teh 'kill target' line of the trigger_once but I'm not sure how you'd go about triggereing that, perhaps use another entity with teh 'kill target' line in it, and just end teh buyzone.
Basically, I'm not sure how CS would react to you 'killing' teh buyzone, but by all means try it.
Just rememebr you'll need to kill the buyzone, and somehow trigger an entity with that 'kill target' line when you shoot the case.