As for drones flying too high, that is a problem but one that could be addressed (not solved) by using interceptor drones to spook or bait the invading drone into flying lower. On a non-ballistic sidenote, you could also have race quads configured to rapidly triangulate the origin control signal and collect video evidence of the signal origin. Not fool-proof by any means, but I wager that would be enough to catch all but the most clever/dedicated attackers.
This all seems a bit silly, but if you throw enough money at it I think you can make it work. The question is how much economic harm can a drone do by shutting down an international airport and how much money are you willing to spend to prevent this. My main point here is that the situation isn't nearly as hopeless as a bunch of soldiers taking potshots at the drones with their rifles (that's pretty much guaranteed to be not work and is exceptionally dangerous.)
Furthermore, imagine you have your dream-drone-killer... how do you even find them? Neither of these recent sightings were even verified to be drones, they just received reports of sightings, therefore initially they would need to deploy devices to find/verify them. Once spotted, it wouldn't be hard to send a couple trained shooters with shotguns loaded with birdshot, assuming knowing their location isn't already enough to find the pilots...