I was thinking about ultrasound or infrasound, but that's dangerous too. A better idea is some kind of organic dirt: about as dangerous as rain, but also disgusting enough to stop all but the most determined types.
You needn't use your real name, of course, but for HN to be a community, users need some identity for other users to relate to. Otherwise we may as well have no usernames and no community, and that would be a different kind of forum. https://hn.algolia.com/?sort=byDate&dateRange=all&type=comme...