Posts can be flagged, though there's a karma threshold required to do so.

A sufficient number of flags will kill a post (it's noted as "flagged").

This is addressed in multiple FAQ items, linked at the bottom of every HN page: https://news.ycombinator.com/newsfaq.html

There is no downvoting for submissions, although I wish there were.

Oh, I assumed for higher karma users they could downvote actual story posts... ohwell