Hacker News really needs some feature to down-vote ... ;-)
You can never down-vote a submission. When you have enough karma you can:
* Down-vote comments
* Flag comments
* Flag submissions
Flags on submissions affect their ranking in a manner similar to down-voting, except that enough flags can result in a submission being deleted entirely.
http://jacquesmattheij.com/The+Unofficial+HN+FAQ#downvotearticle
[0] https://news.ycombinator.com/user?id=jacquesm