If you shield a drone it becomes heavier and then needs to be bigger. Also at that point the drone needs to either be self guiding or have a communication/control system that won't be knocked out. You get the old little more weight little more propulsion to carry the weight cycle going and all of a sudden your drone isn't small anymore.
So what's your point? That a more capable drone is also bigger? So? So what? That's obvious. Do you think the added size/complexity of such a thing would be any impediment whatsoever to a determined aggressor? If you want to penetrate controlled airspace to do something nefarious, you're perforce going to want something that's difficult to detect, difficult to jam, difficult to shoot down, and has enough payload to carry whatever you need to cause the damage you're looking for.
That seems an incredibly strong statement. So strong that it looks like it doesn't have enough thought behind it.
Really? Then let's hear your alternative options. I already covered sensors and weapons, but let's recap. Radar is vulnerable to stealth, so it won't do the job alone. Lidar is too short ranged to do the job alone. Acoustic is even worse. But put together, a web of such sensors would be very difficult to overcome. If there are other sensors out there that are even remotely applicable, please enumerate them.
As for weapons, you have only three options: ballistic, missiles, or directed-energy weapons. Ballistic weapons have all kinds of downsides, from trajectory computation to wind to limited ammo, not to mention the inevitable collateral damage from misses (of which there will be MANY). Missiles have similar downsides. DEW's have (almost) none of these, the sole one being the potential for (minor) collateral damage in the case of a miss. You could even potentially mitigate this by using an array of low-power lasers, individually almost benign, but focused together to take down a drone.