You could downvote the submissions but I think the main benefit of HN is in the discussion and commentary. I personally read the comments before I click on a story because I like to have a frame of reference when choosing to read an article. I don't think a post should be ranked on its upvotes but on its number of comments and range of discussion.

I should also add that just because a submission is bad doesn't mean the discussion under it isn't worthwhile.