Assuming you know the frequency a drone is flying on, it is trivial to jam. However a lot of the more advanced flight control systems (even on hobbyist drones) will have a failsafe that causes it to return to home or similar in the case of loss of radio contact. I suppose even in that situation you could use a GPS jammer to stop that from happening. Of course this would all be highly illegal.
Yep, the vast majority of 2.4 GHz hobbyist RC aircraft do this, but you're still subject to noise floor issues. It might take a heck of a lot of power, but there's no technical reason why this stuff still can't be jammed, and it's not that rare to have signal drop-outs caused by (presumably) unintentional interference.
Powering up a multi-megawatt 2.4 GHz jammer will get you arrested. You are not allowed to transmit in the 2.4GHz band over a few watts (and even then) without a license and the rules are very specific for what you can send (in Australia for example you're not allowed to encrypt that traffic).
And frankly, all the panicky idiots who keep salivating at the prospect of wrecking things makes me glad the rules for radio transmissions are the way they are. Heads off a heck of a lot of problems.
My favorite comment was the user who was so worried about the danger of drones falling on people in Yosemite that he/she would throw a rock to knock the drone out of the sky. Solid logic.
You don't need multiple megawatts. You just need to be louder than the original transmitter. And that's not all that hard, especially if you can use a directional antenna pointing in the direction of the drone.
Being 'dirty' is a lot easier than being clean. The bands are typically not all that wide and to mess them up is easier than to be nice. A sawtooth modulator with a suitably high frequency modulating a carrier will disrupt a significant chunk of the spectrum with reasonably little power input. After all you only need to disrupt part of each packet to disrupt the packet. Jamming is a lot easier than getting a nicely modulated signal through.
Hardware for autonomous flight will have to include both GPS and inertial sensors. I suppose an EMP beam could theoretically be used to damage the electronics, but it probably isn't very practical and the electronics could easily be designed to shield against this if it became a legitimate threat.
