Next week: App that emits frequencies that block Flock microphones

2 weeks from now: Google and Apple remove AntiFlock apps; developers arrested

(Only slightly sarcastic. We should be talking about ways of making these things useless, as well as illegal)

EMP devices.

1. This is illegal eavesdropping

https://www.law.cornell.edu/wex/plain_view_doctrine_0

might apply, though IANAL

That's probably exactly the rationalization for this. No expectation of privacy in a public space, and therefore no requirement for warrant to eavesdrop. I may not like it, but the government will find absolutely any way to do things we don't like in spite of the infinite attempts to restrain it from doing so decades or centuries ahead of time.

There are eavesdropping laws specifically about audio recording. Not sure if it's federal or state by state.

Get little throwable "annoy-o-tron" devices with a button-cell and a magnet that randomly play little gunshot sounds or fake screams and throw them near the Raven sensors.