As a determinist who believes that libertarian free will is an illusion, the argument over whether we have libertarian free will or not is somewhat passé. The interesting debates happen over whether we have moral responsibility or not, what any ramifications of this would be, and what approaches we should have to crime and punishment.
It is on this last point that I would like to dwell. Some philosophers and thinkers espouse using deterrence as a justified method of punishing criminals: that harsh punishment can be justified as deterring others from committing similar crimes. However, there are problems with this, not least using people as a means to an end. Immanuel Kant, for example, was against using people instrumentally; there are problems involved with inhumanely treating one as a means to creating a greater good for others.
I would like to park this idea for now and concentrate on another approach to punishment: quarantine. This is really well set out in Derk Perebooom’s excellent book “Living Without Free Will”.
This idea has a foundation in realising that it is morally acceptable, if a person had a highly contagious disease, to quarantine them for the benefit of others, but also for themselves. This quarantine should be as humane as possible and should also concentrate on getting the person better, on rehabilitating them. This person remains quarantined, humanely, until such time as they are safe to reintroduce into society, until their rehabilitation is complete.
This is the same for criminality. Using the same logic, it is morally acceptable to quarantine such criminals for the good of society, whilst treating them as humanely as possible and working hard to rehabilitate them. Upon the event of the person being rehabilitated and safe to reintroduce back into society, this should be done. However, there might be cases where the quarantine period is indefinite or very long indeed as rehabilitation might not be forthcoming.
This approach concentrates resources on rehabilitation and not retribution, as there is no space for retribution under determinism. It is humane. It also keeps society safe from harms.
Probation services have known for a long time that the prison systems should be about rehabilitation, as recidivism (reoffending) rates will never be reduced without it. Over recent decades, however, I think we have lost site of this as locking criminals up has become a political football seen to play on the fears of the electorate, and rehabilitation is seen to have higher costs.
From a moral, desirable and, in the end, pragmatic point of view, though, the quarantine approach is the most justified mechanism to dealing with criminality.