A mug friend posits that it isn't the private part of the rapist that's the problem. It's the brain. So how about we LOBOTOMIZE RAPISTS?

Not sure that would solve the problem.

Following the operation, spontaneity, responsiveness, self-awareness, and self-control were reduced. The activity was replaced by inertia, and people were left emotionally blunted and restricted in their intellectual range.

You may create someone with less self-control, and who has less emotions (so less sympathy / empathy with others).
Perhaps turning them from a rapist to a psychopathic killer.

Unless they are a masochist ... I would say if we want to get violent, do unto them as they did unto others.  Tenfold.