If people type in the comments and they get close to what will actually happen I will often reply with something like "we'll see! :3c " It's a bit of a tease without being a full blown spoiler.
But if people are way off I usually leave those alone. Especially if it sounds like their minds are made up that what they predict is what will happen. I won't correct them in the comments, I'll just have to wait for the story to get there.
Otherwise, I spoil everything to a third party, namely my husband, who doesn't have a vested interest in the story, but has a lukewarm interest in my creative endeavors. I use him to bounce ideas off of all the time.