Its a unlocked feature once you have enough points

How many points?

Here's a decent write up

https://github.com/minimaxir/hacker-news-undocumented

For posterity, you can only downvote comments, you can't downvote posts.

It doesn't say anywhere in that document that you can downvote posts.

For comments only, not posts. Downvoting comments will achieve nothing when it comes to reducing the prevalence of AI spam posts.

You can flag posts though. Enough flags kills.