How many times have you heard someone say "he should be castrated" of a phedophile or rapist. Are people so niave as to think the sexual predator will not creatively find another insertion device. After all, it's more about control and sadism than sex.
You really want to solve the problem. Well, madatory death penalty for certain crimes would do it (assuming we can trust our legal system to prosecute on the "no shadow of a doubt" theory, which I think we all know would be a faulty assumption). There is jail, but how many times has one (oops) been let loose or let loose as they have been "rehabilitated" supposedly. Only for some unfortunate soul (normally a dead sole as the perpetrator does not want to go back to prison) finds out later on.
Assuming we could trust our legal system as above, I say take off all extremities. Arms, legs, and then you could leave the sexual organs be. Like to see someone try to rape or molest another with no arms or legs. Alright, pull out their teeth too, just in case.