Those people can upvote other stories they think are more relevant or interesting. The "newest" page at http://news.ycombinator.com/newest always has interesting submissions waiting to be upvoted.
Not all users can flag, and flagging is more to deal with spam and trolling. (The guidelines mention offtopic posts as well, but community consensus is that flagging is not downvoting.)
Flagging is not downvoting, but it is the sanctioned way to register your opinion that some content does not belong on the site. If your efforts to combat off-topic posts are limited to ranting in the comments after they're posted, it's too late.
Well, then you have an issue not directly with this story but the way HN is designed. Start a submission of your own, citing this example, about this issue and get the discussion rolling, if you feel passionately about this.
