"Jamming" is legal in as much as any device that operates in the ISM[0] bands must accept interference - so in the 2.4GHz example, microwave ovens, baby monitors, TV relays, remote door monitors, wireless alarm systems etc., all operate in the same band and will interfere with WiFi that is close by (since they use a different modulation).
Definitely start with the presumption it is illegal anywhere in the world, even though you might be able to hack around regulations to find loopholes. I think the FCC is focused on broadcast interference, safety requirements, and commercial sales of jamming devices.
And in college I remember a professor or a TA demonstrated jamming by operating a modified microwave oven while running iperf or something similar on a nearby Wi-Fi network.
[0] https://en.wikipedia.org/wiki/ISM_radio_band