Popa is 100% correct. And a punishment cannot be applied to an individual where the Torah doesn’t allow it, even if the purported purpose is to benefit society.
And Sam, halachicly that is only the case if there was an actual maaisa. Otherwise you are incorrect.
As far as protecting innocents, absolutely. But it cannot done in a manner that penalizes the person in a manner unauthorized by the Torah.
And, no matter what, once he served his time he has to live somewhere. And you cannot tell him to go live next to yenem so he is further from unzerer. (Kind of like no one wants the incinerator in their neighborhood — but its gotta be somewhere. And your neighborhood is just as qualified as the next guy’s neighborhood.)