Flag them, enough flags kill the comments. After a while, the account/user is banned. If you see too many that are undetected after a few days, you can email dang/tomhow hn@ycombinator.com

For the obviously stupid comment I'd not worry to send an email, until there are too many.

For the last one https://news.ycombinator.com/item?id=47130567 perhaps you can send an email just in case.