On the one hand, it's correct that it's a public forum and you are free to post responses to any thread and to anyone you want.
On the other hand, don't you think that it was egging him on to keep making replies on threads he created? Yes, you have every right to reply to his threads, and just because he said he didn't want you to, it isn't anything you have to comply with.
BUT.....the fact that you kept answering his threads seems like provocation to me. You're the one who started harassing him by doing that after he has asked you to just leave him alone and not reply to anything he posted.
It's not like he wanted you to just leave the entire forum -- he only asked you to avoid his particular threads so that the two of you just don't even have to deal with each other, since there is no blocking feature, you say.
Now it's all escalated and it didn't have to if you had just kept away from his individual threads he started.
I'm not taking sides, I'm just saying this has all escalated and he's mad now but only because it was you who didn't leave him alone in the first place.
Don't you think all of this would not have happened if you had just ignored him and any of his threads from that point? It would have turned out better if you had.
That would have nipped any further contact in the bud, since it seems it's initially all he asked and wanted.