Please add an IP blacklist. Vistors with IPs listed in that blacklist wouldn't even see the comment field or the comment reply button.
The comment blacklist should be a textarea where you can list black IPs one below each other like so:
xxx.xx.xxx.xxx xx.xxx.xx xx.xx
It's unbelievable, but neither WordPress itself nor any plugin offer such an IP blacklist for comments.
Settings-->Discussion-->Comment Blacklist is something completely different.