I don't think cookies will do much against a large-scale automated attack, but everything else in this list is solid:
Hide posts until legitimacy of the poster has been verified. Allow them to post and respond, but don't show it to anyone yet, except to the moderators. If they're posting something sensible, unhide them. If it's spam, shadow-ban them. Don't let the user know. Let them guess why nobody is responding to the spam.
For that reason, it may also be a good idea to post an announcement that nobody should respond to this spam. Tat way the spammer won't know if he's being ignored manually or auto-hidden. Let him waste time and frustration on that.
Only use this against people who are this malicious. For regular hot-headed people who accidentally break forum rules but do want to meaningfully contribute to the community, always remain open and honest. Give people the opportunity to learn. Only people who are determined not to learn and to remain purely destructive, do you use the shadow ban.
Of course once they catch on, they'll probably start making new accounts with some legitimate posts, and once people start responding, they go back into spamming mode. This is tricky. Ideally, it'd be nice if you had a system that could automatically detect that sort of spam. If someone suddenly starts posting ascii art, bigimages, all caps, or anything like that, or goes on a rapid posting spree, automatically put them back on probation with hidden posts requiring approval, so you can check this change in posting behaviour.
Hide posts until legitimacy of the poster has been verified. Allow them to post and respond, but don't show it to anyone yet, except to the moderators. If they're posting something sensible, unhide them. If it's spam, shadow-ban them. Don't let the user know. Let them guess why nobody is responding to the spam.
For that reason, it may also be a good idea to post an announcement that nobody should respond to this spam. Tat way the spammer won't know if he's being ignored manually or auto-hidden. Let him waste time and frustration on that.
Only use this against people who are this malicious. For regular hot-headed people who accidentally break forum rules but do want to meaningfully contribute to the community, always remain open and honest. Give people the opportunity to learn. Only people who are determined not to learn and to remain purely destructive, do you use the shadow ban.
Of course once they catch on, they'll probably start making new accounts with some legitimate posts, and once people start responding, they go back into spamming mode. This is tricky. Ideally, it'd be nice if you had a system that could automatically detect that sort of spam. If someone suddenly starts posting ascii art, bigimages, all caps, or anything like that, or goes on a rapid posting spree, automatically put them back on probation with hidden posts requiring approval, so you can check this change in posting behaviour.