Ambrose did his best to keep his face emotionless, though it did twitch a bit at the last comment. "Finnigan," he called after him, keeping his wand pointed at the other boy's back, "You've just earned yourself a shadow. I'm going to watch you so closely, you're going to start getting creeped out. And the moment you step even a toe out of line, I'm going to the Carrows, and you'll lose your prefect badge before you've even realized what's happened. And then I'll start watching your friends, and one by one, I'm taking them down, too."
Lowering his wand, he shot one more look at him before pocketing it. "And don't think I won't."