It wasn't easy. On about day 30 or so I destroyed all farms, then a few days later I realized it would be wise to destroy my housing as well, to prevent immigration. Then it was just a matter of letting the game run... and run... and run... Cults were formed, people went in and out of madness, but starvation was rare because they kept foraging and hunting. Day 67 finally took care of my last stubborn survivors. This was on the arctic biome btw. But wait, that's not all! Then I left the game running and there was a foreign invation of... nothing. I still managed to get the Mecharepublican troops to help though since we were friends before the extinction. And the empire also sent some, which brought my population back up to four until they got slaughtered.