system
A chat on subreddit /r/NoStupidQuestions
human
As an extension on this, the upvote/downvote system that reddit uses is brilliant. Comments with a negative rating are automatically hidden.
gpt
Though it is specific to comments and not accounts, it should be noted. If your account has a lot of karma but your comment has negative karma it gets hidden. Conversely, if your account has negative karma and your comment has positive karma, it gets shown (assuming there aren't a lot of other comments in the thread).