The cockrings wouldn't be too nasty. No permanent damage... but tight enough to hurt and she'd make them wank for her benefit, knowing they wouldn't be allowed to come. :D

And there will be no hair loss! I actually have a whole theory about that, considering how important hair is in the Potterverse. She wanted to make him impotent, and it's one of those visual jokes she's into. *glares and hugs Draco* He is still attractive!! The Draco fangirls shall not be thwarted!

Male pattern baldness is descended through the female line, though: Cissy's father would have been thinning on top, too.