"I didn't post this to start a debate about..."
You posted on a public forum, the other users are going to engage with it how they will within the rules of the forum. You don't have to engage them back but you don't really get to dictate how they engage initially.
If you just want a public digital space to post your thoughts without any feedback you can just start a Twitter and mute replies, or a blog.
I think it depends on the post, surely. Sometimes people are not posting for the sake of discussion, but to ask a community for advice about something that effects them. In that case to derail their post in a way they believe distracts from that would be unpleasant. But I do get what you mean when the post is about something less personal, and the lines do get blurred.