How can you impose penalties for something you just don't like (streaming), without creating settings to address it? The way you make it sound, he's playing within the rules and simply trying to win. If you limit the # of moves managers can make, then the streaming issue takes care of itself. Why not set the league up this way instead of "warning" people? Otherwise, how do you draw the line? What's considered "streaming?" Three pitchers starting in one day? Well, what about three in three days? Or two in two days? What about if Westbrook starts to suck, and I go pick up Volquez but he happens to start the next day. Is that streaming? Can a manager pick up ANY starters? Ever? Is it only streaming if you pick up starters when you are behind and trying to catch up? Please don't tell me you know it when you see it, because that just won't fly. My point is, you have to define it, or you will have arguments. You have stated to the league that they cannot stream, which has some sort of measurable pickup/drop (to you). So...why not just put this in the rules to begin with? Talk to the league, and come up with a "move" cap and add it to the settings (if possible).
I don't see how you can penalize him. Just my opinion
And for that matter...why are people so against streaming, anyway? Another interesting part of fantasy baseball. And anyway, it often doesn't work (ERA, WHIP), and it can be countered.