Assume they’re already in the worst box that various legislation (mostly related to human rights) permits you to construct, or the closest cost-effective approximation thereof.
At that point, if they are not already, they should be put into solitary. Some would consider it reward, but if they prey on others, then they should be put somewhere that they can’t—that’s (ostensibly) why they’re there in the first place, at least in part.
Put them in a smaller concrete box and with other prisoners that lower that estimate of their lifespan?
Assume they’re already in the worst box that various legislation (mostly related to human rights) permits you to construct, or the closest cost-effective approximation thereof.
At that point, if they are not already, they should be put into solitary. Some would consider it reward, but if they prey on others, then they should be put somewhere that they can’t—that’s (ostensibly) why they’re there in the first place, at least in part.