Rehabilitation does not necessarily require punishment, and there are reasons for punishment that are not based on rehabilitation.

It is possible for someone to be rehabilitated on their own as they change, or through someone taking action that is not punishment (explaining the impact of the actions which the person may not have been thinking of, etc).

It's possible to punish someone with no thought as to rehabilitation. Capital punishment is this. The purpose is not to benefit the person in any way, but to benefit society by providing a negative example to others so they don't attempt the same actions (it doesn't matter if you believe it works, this is the purpose it's done).

I would suggest prison as a way of preventing them from doing it again. not to harm them but to prevent them from causing harm.

You have it backwards. The punishment is must be there as a deterrent, regardless of the possibility of rehabilitation.

Otherwise nobody would care whatever a judge decides.