There's a value to a Hell's Island or a Gitmo for such had cases.
Edit to last "No prison rehabs such men" the gang culture is full of ex-cons and cons still operate in the gang while in prison. Prisons BUILD the strength of the gangs as far as I understand.
Well, if they're convicted it's OK with me if we just shoot 'em.
Hmmm ... if the level of violence is great, yes they could be shot under Natural Law principles — just for wearing gang colors while a disturbance is underway, or if making a threatening move!
It’s a form of war and terrorism, like piracy. Any act that identifies as person as a member of such a terrorist group makes that person an enemy combatant, subject to that which is done only in war — killing the enemy.