Once you accumulate a "small karma threshold" (quote from the HN FAQ) you can 'flag' submissions. Flagging is the closest to a "dislike" option available. But it is suggested to use it for items that are off topic.

The better way to make sure items you find to be "positive" make the front page is to upvote everything you think deserves to get there. Upvote the positive, ignore the negative, and the negative will fade from view.

https://news.ycombinator.com/newsfaq.html

[edit]: add additional clarity