a) Anybody can flag a story, and that somehow causes spam to be flagged in automatic fashion.

b) A few users are given moderator privileges and the ability to mark stories dead or users ignored. When a story is marked dead it is visible only to the submitter. When a user is marked ignored all his stories are automatically marked dead.

There are others that are secret to us users.

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Further reading:

http://ycombinator.com/newsnews.html

http://paulgraham.com/hackernews.html

http://www.joelonsoftware.com/articles/BuildingCommunitieswi...

http://www.codinghorror.com/blog/2011/06/suspension-ban-or-h...