I honestly don't see the issue here, if I don't like somebody, I don't understand why I'm obliged to play with them on my own server
You're not. But it's rather dumb to ban someone from your server because they disagreed with your opinion on the forum, but to not ban someone who joins your server, insults your work and calls you names.
Like, why wouldn't you ban both? Or ban neither?
Unless you have an ulterior motive, which is to create this faux sense of exclusivity, and make it some sort of punishment for being mean to the guy on the forum.