I feel like this is a problem some smart people could solve quickly if they were asked, similar to the healthcare.gov situation. Ask some MIT students to come up with a solution over a weekend and I bet they would deliver.
Knocking a slow moving object out of the sky is literally not rocket science.
It gets a little more complicated in the real world if you don't want to shoot down the wrong flying objects and also not kill people on the ground.
No, but it's not a matter of some yahoos with a gun shooting them: https://twitter.com/sommervilletv/status/1080724993238069249
Meanwhile in Kentucky: https://arstechnica.com/tech-policy/2017/03/judge-rules-in-f...
Meanwhile in Virginia: https://arstechnica.com/tech-policy/2016/08/65-year-old-woma...
Meanwhile in Tennessee: https://arstechnica.com/tech-policy/2017/04/man-takes-drone-...
And for the unaware, birdshot solves the "what goes up must come down" problem; the shot is light enough that when it falls back down the air resistance has slowed it to a harmless speed. In other words, the terminal velocity is MUCH lower than the muzzle velocity. Getting hit by falling birdshot doesn't bruise, let alone kill. If somebody were serious about solving this problem, instead of just trying to justify impotent anti-drone legislation (that won't stop bad actors from throwing together rc quads from scratch) they'd ask gunsmiths to create for them a fully automatic shotgun tuned to cycle with birdshot, with a large magazine capacity, and create radar systems that can distinguish between birds and drones (probably with a human in the loop.) Anti-drone micro-CIWS.
Other promising solutions include anti-drone falcons (potentially hazardous to the bird, I've seen what those props can do to human fingers...) and hunter-killer drones piloted by superior pilots. A good defense would be a defense in depth, adopt as many of these systems as is economical. Considering the economic harm that shutting down a major airport can cause, the budget should be large.
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...
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.)
But perhaps doing it in a way that doesn't shower the runway with dangerous debris, is?
Met a guy while he was on the clock doing that, in fact.
A bullet or other kinetic object could do damage if it falls back down to earth. A directed energy/RF weapon could possibly fry other electronics in aircraft nearby as well.
This [1] is a handheld one, though automated permanent installations that use acoustic signatures to track drones have already been installed on embassies and such.