Wrap it in foil and stick it in the oven!
> I also tried putting my phone in a (turned off!) microwave, but this did not work either, it was still connected to the Wi-Fi access point.
But yeah I would have thought the same.
Never checked if true. But that could explain why they're so "leaky" at 2,4 Ghz (and other frequencies).
Apparently modern wifi chips are just too good at picking up faint signals.
Also when a phone is in a microwave, the RF noise is also attenuated, so that helps a little (amplifier noise is unaffected).
But that's not even close to sufficient for actually isolating wireless transmissions. My microwave got 35-38 dB for WiFi signals when I measured it, and that got through a fairly reliable connection. You might want 90 dB or something like that if you want to ensure that you can't extract the data from the noise - so perhaps a starting point might be to put your device in a small microwave oven and put that oven in a large microwave oven :)