Dupe: https://news.ycombinator.com/item?id=44134873

The other post was flagged, for whatever reason.

Flagged =/= dead, FYI Once you rack up enough karma you can also vouch for a flagged post.

How does this work? How does one know if they have enough karma to do so – like where does it show up in the UI?

It boggles my mind that this doesn't already exist.

It shouldn't exist. Some things shouldn't be made. It's our job as decent people to pointedly not make them due to their inherent harm potential.