Sam doesn't look at Emily while he delivers the pitch, still in the same bored tone. "You want to punish her for what she did, right? You could do that - mark her up so the bruises are a reminder to everyone else - or you could do one better. Everyone ignores her, everyone stays away from her, everyone keeps her out of their circles. Because the alternative is crossing you and getting the same treatment. Bruises heal, but this lasts. Once it gets around, it sticks until you let everyone know you changed your mind. Best part is, you don't need to lift a finger to enforce it. She'd be a walking reminder of what happens if you two-time Roberto. She just...stops existing until you say otherwise."
It's a long shot, but there's a chance Emily could find her way back into good graces with the jackass eventually - and after all, it's not like Roberto has supreme power in the school. Better to leave the door open for things to change in the future than risk Roberto blowing his fuse now and doing something more permanent.