Sex Offender Registries cause more harm then prophylactic success. Neither are they completely effective.
In the past, in a biblical manner, kings simply cut off the offending organ set to prevent their guards from screwing the women of their harem. It seemed to have worked, been effective; even is described by Jesus Christ, although to lay down an advanced heavenly meaning.
It should serve the purpose of prevention of future offenses, as well as be a valid classification label along with the likes of "transgender", "gay"...: "Eunuch".