For anyone like me, unable to figure out how to do that - you click Reply, and then there's a 'flag' link.
The temporary solution is to shadow ban the comments, the usernames seems to follow 2 naming schemas. Banning them completely will alert the attacker to change the naming schema, or to make it more random, which will make stopping them even more difficult.
I'm honestly perplexed that HN doesn't have any kind of string filtering facility considering its centrality in the tech ecosystem.