Keep it civil, do not flame or bait other users. If you notice anything illegal or inappropriate being discussed, contact an administrator or moderator.
Well, it's been a thing since before I became a mod, which was about five and a half years ago.
This isn't the type of policy that's likely to make it into the ToS, but I'll definitely propose making a change to the howto:comment wiki entry to make note of it to the mod team.