What should I do about my previous posts on an anime forum, especially after regretting the incitement or insults I directed at one of the young men, given that I cannot delete my many posts nor can I apologize to him?

If there is anything forbidden in the topics you have written, you must delete it if you are able. Otherwise, there is no blame upon you, for Allah Almighty says: "Allah does not charge a soul except [with that within] its capacity."

As for the rights of those you have wronged: if you can seek their forgiveness and pardon, then do so. Otherwise, seek forgiveness for them and supplicate for them.