Punishments meant to deter crime only work if criminals are anticipating facing consequences of their actions, and they're not desperate. Increasing the maximum penalty from 10 years in prison to 20 years isn't going to make someone think twice before robbing a petrol station to feed his starving family (please note that I'm making up both these circumstances and their associated punishments - I have no idea about specifics, I'm just talking principles here). Or for someone who doesn't think they'll ever get caught, potential punishments are meaningless. Your only choice is to take a course of action that rehabilitates that individual after the fact or, better yet, pre-empt the problem and prevent the circumstances that lead to the crime in the first place.
Mokalus of Borg
PS - I have no authority to speak on this subject at all.
PPS - That should be a standard disclaimer on something like 95% of my posts here.