We nearly always inform people when we ban their accounts: https://hn.algolia.com/?sort=byDate&prefix&page=0&dateRange=....
Lots of software affects both story and comment rankings. "Poison" is a rather tendentious way to describe that. HN's goal is high quality in all things, to the extent it's possible. That doesn't just happen by default; systems are necessary, and it gets harder as numbers grow.