Start a game as a Lithoid Empire. I was playing as a Xenophobe/Authoritarian/Militarist Necrophage (so something in that combo would work), but you could also play as a Hive Mind (Gestalt Consciousness). You just need to have some Ethic aggressive enough to allow you access to the most violent diplomatic options.
Join the Galactic Community, get a few resolutions passed that are easy to break (Military Readiness is an easy one), and wait to see the little red icon at the top of the screen that tells you that you are in violation of Intergalactic Law. Ignore that icon.
After a period of time (I was playing on fastest speed, so I'm not sure exactly), you'll be sent a notice from the Galactic Community that reminds you to follow the law. Choose the "Eat the Envoys" option and the notice should pop.