Streamer Trolls

This is the first time this has happened to me. I was playing The Witcher 3 a couple of days ago and having some very interesting discussions with my viewers when someone popped in and dropped two spoilers in one post and then appeared to leave the channel. Why do people do this? Do they really have so much time on their hands that they would rather drive by troll then say play a game or actually have a real conversation with someone? Anyone have a suggestion on how to prevent this in the future? I mean I can post rules saying NO SPOILERS in ultra caps but I feel like that won't make a difference for the drive by trolls.