We should touch on the point that suing people over emotional slights experienced in online activity, is a fruitless endeavour. Further, entertaining fantasies about suing people, is hopeless, and ultimately demoralizes and weakens the thinker.
Platforms themselves have broad immunity to being sued due to the conduct of their users, under a law called DMCA Safe Harbor. This would be their first pleading, and your lawsuit would be stopped right there.
However, even if Safe Harbor didn't exist, TOS is a contract between them and you. Any valid contract has an exchange-of-value. Your value is getting to use the service. Their value is you giving away certain legal rights. (Typically things necessary to running of the forum). All this to say, TOS is you giving up rights, not taking any.
Both DMCA and TOS are designed to protect the platform. This is important because if platforms powered by user-generated content couldn't control their legal costs, nobody would create such platforms, and we wouldn't have any. All that to say, they don't protect users from each other.
Usually, your only viable of action is to sue the other user directly. (To use the word "viable" loosely). But you are limited by how much damage you can actually prove. Between two pseudonymous figures on an internet forum, it will be difficult to prove any meaningful dollar value, except unless the identity you're attacking has developed a cash value.* You might get more traction with a court order to compel or restrain certain actions, but even then, you're arguing a balance of rights - "your" reputation vs their free speech.
Maddeningly, a foreigner's free speech rights in a domestic forum will be firmly recognized by domestic courts, but your right to collect money from them will not, on the basis that they're are out of the court's jurisdiction.
* for instance if I claimed PewDiePie was a Nazi war criminal, that is a developed brand with a lot of money at stake, so they could easily show money damages.