Punishment is logically for a crime committed. That the crime 'physically cannot be committed again', makes no difference. On a retributive theory of punishment, punishment is merited simply by virtue of the crime's having been done; neither reform nor deterrence is relevant.
A reformative theory of punishment would still apply to the case you describe. There is punishment because the crime has been committed and the aim of reform would be to prevent the criminal from engaging in similar crimes or to discourage her/ him from law-breaking in future.
Reform does not replace punishment here; it is the aim of punishment.