What a hatemonger wants most is attention. He thrives on being able to push your buttons and get you to react to him. That will never accomplish anything except make the hatemonger want to post more hate.
The ONLY way you can try to deal with this is to ignore. And it will take all of you to do the ignoring. If one or two don't have the willpower to ignore and keep reacting then it will continue until he gets what he wants and kills the forum. And it may already be too late because he's driven all but a handful of others away already. But that is your only hope. If you ALL can have the willpower to never reply to any of his posts eventually he'll get tired of it and go away. And you can hope by then there is still a forum to save.
The ONLY way you can try to deal with this is to ignore. And it will take all of you to do the ignoring. If one or two don't have the willpower to ignore and keep reacting then it will continue until he gets what he wants and kills the forum. And it may already be too late because he's driven all but a handful of others away already. But that is your only hope. If you ALL can have the willpower to never reply to any of his posts eventually he'll get tired of it and go away. And you can hope by then there is still a forum to save.