I’ve had problems with a few very stubborn attackers.
Look up the “gray rock technique”. I found it best to not reply or respond to them, because they often just want some attention. Don’t talk about the user publicly (where they can read it) or acknowledge their existence. If no one responds with anger and outrage and their content just gets quietly deleted and banned, they will eventually get bored. It’s still difficult though. The worst one ate up about 40 hours of my time one week before they finally disappeared.
If you go to this dashboard URL /admin/customize/watched_words/action/block you can add words that will cause a post to be held for moderation. Look for the “Silence” item in the left menu: “First posts of users containing these words will require approval by staff before they can be seen and the user will be automatically silenced.”
There’s also this: Google Perspective API Plugin