Given that the cost of administering the punishment would be worth paying to prevent the crime...

You should punish them if they are the kind of person that before they do a crime, accurately consider

• whether being the kind of person that doesn't do the crime implies they should have avoided some punishments in the past that they in fact did receive.
• whether being the kind of person that does the crime implies that they should have received some punishments in the past that they in fact did not receive.

And then weigh the gains from the crime against the against the losses of the punishments they should have received or and the gains from the punishments they should have avoided.

OR

If at some point they considered whether to be the kind of person that does the above, and decided not to because that would make them susceptible to punishment of future crimes.

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I think the above conditions are finally sufficient, but not necessary. Some other kinds of agents are worth to punishing for their future crimes too.

Also, the question I was answering above was not really "when should you punish someone for their future crimes", but "what kinds of people are worth punishing for their future crimes". Maybe that's why the answer is so long.