If you've already explained why you don't like Halloween then he's just being clueless. I get (perhaps childishly) enthusiastic about Christmas and sometimes want eeeeeeeeeveryone to share it with me, but DH is just not as into it as me. He doesn't dislike it, he just has less enthusiasm than me. He very sweetly says that he enjoys seeing me enjoy it, but I think that would fade fast if I was pestering him twice a day about it.
How you address this really depends on your communication style within your relationship
. Personally I would go with "Sweetie, I need you to listen to me now. Do I have your full attention? Great. Now, it is very important for you to understand this because if you ask me why I don't love Halloween even one more time I'm going to have to kill you with an axe. 'Kay? You've asked me that question twice a day for the last week and I've really, really gotten over hearing it. You run along and put up more decorations now, but don't forget! Love you
" And then I'd pretend I was looking for my axe any time he slipped up. Obviously, though, that approach would be seen as grounds for genuine spousal homicide in some relationships
. So in short, just TELL him, in whatever way you know will get through, to stop asking you.