Count me as one of the people like prefer to be surprised than read ahead. I generally just trust that the author has a plan for the particular way that they lay out a story and it will be a better story for me if that is how I experience it.
In my experience, people like the person you talked about think that they are doing everyone a favor by "telling them the things that no one else has the guts tell them." It usually only takes a time or two of pointing out to them that they are really doing more harm than good and no one is better off for their advice to get them to stop. It's especially effective to you catch them doing it and point it out to them just at that moment.
Dear Diary,
Two weeks now since we've heard a peep from Dom. And yesterday, I saw a snake with 5 heads. I am relatively sure that these are both signs of the impending apocalypse.
J-Dub