No one can down-vote posts, but posts that require attention (moderator related?) can be flagged to indicate that the link is to something that is outright bad (spam, inflamatory content, false information etc) and if it gets flagged by a lot of people it falls down the front page.
There's probably something else I don't know about the ranking algorithm though.
I lost my ability to flag. I have no idea why, I assume I flagged one too many posts that ended up as popular. Just as well. One less distraction for me.
I don't think so, not for just flagging but there does seem to be a way HN can find this type of behaviour and ignore the actions of users that abuse the voting system. read this for some more info http://www.jacquesmattheij.com/The+Unofficial+HN+FAQ
There's probably something else I don't know about the ranking algorithm though.