Sally uttered the scourgify spell, which helped to some degree, but still left a stain. It was not unusual for her spells to be a bit weak, especially when she was frustrated.
But she was wise enough to acknowledge the fact that Justin seemed to be retreating. Was it something she had said? Perhaps the muggle part didn't sound quite right, when it was really the post part she was lamenting about. Not to say that she understood muggle fashion in the slightest. Their clothes were always so weird.
"Thank you, though. For being nice this time around." She say no point in burning bridges unnecessarily.