An idiot cull is a practice whereby users are told not to do something, and any users who do it are permabanned. Sometimes the threat of a permaban is even made clear. They were carried out several times during Hezzy's administration and were very popular, with users often requesting "another idiot cull" in response to bad threads.
The first idiot cull took place on January 9, 2008. Hezzy posted a thread in General Discussion entitled Please don't post in this thread. The thread reached ten pages, and dozens of users were permabanned.
Another famous idiot cull took place at some point in 2008, when Hezzy posted a poll thread in GD, with an option labelled "Comedy hurr option that gets you permabanned." The resulting wave of permabans confused many users who were unaware of thread, and some believed it was the result of a new Facepunch policy where bad jokes would earn a permaban. Mojo Week was widely mistaken for an idiot cull, to which Hezzy responded that he only permabans people "when they make mistakes."
Another famous idiot cull was in early 2013, right after garry closed down Ask us more about the forums and started to discourage megathreads. Most users who posted in the thread were banned for the reason "culled", while Spencer Sharkey was banned by every single mod, and Brochacho unsuccessfully attempted to evade punishment using a hastily-drawn Ban-Proof Shield. Around the same time, garry closed a thread complaining about the "no megathreads" rule, and said that people could "rate this post DUMB to be banned." People who rated the post dumb were mostly banned for the reason "culled" although some appealed successfully and got reduced to a month. 
- Xtimeline - event timeline [external link; broken]: http://www.xtimeline.com/evt/view.aspx?id=222464
- Sensationalist Headlines/Polidicks - "Mojo Week is OVER" (post #130): https://facepunch.com/threads/916790?p=21121763#post21121763
- General Discussion - Do not post in this thread: https://facepunch.com/threads/1248593
- General Discussion - "Do not post in this thread" (post #36): https://facepunch.com/threads/1248593?p=39681775#post39681775
- General Discussion - "Do not post in this thread" (post #42): https://facepunch.com/threads/1248593?p=39688979#post39688979
- General Discussion - "So, how is this "no megathreads" rule going to work? (THERE IS NO RULE IS THE ANSWER TO THAT)" (post #222): https://facepunch.com/threads/1250086?p=39735154#post39735154