It takes, iirc, 500 karma to be able to flag an article. So it would take 5 users with 500 karma each to knock it off.
It'd take a lot more effort than it was worth for a sockpuppeteer to do it, but I bet any company with 25+ employees could knock an article off the front page if they caught it early enough, and were able to marshall enough people to do it.
That, and I'm nearly certain HN has algorithms that will pick up on a group of people consistently flagging things together. PG hinted at it a while back.
