HN already has a rate limit. You get the message "You're posting too fast" when you exceed it. Though I think it's also tied to karma and might not trigger when all those comments get upvoted. And if you try to downvote them all, you'll run into the protections against mass-downvoting. Tuning the feedback loops to encourage good discussions while damping flamewars is a difficult problem.

And they should ban throwaway accounts too which you can also see spreading more conjecture.