A simple middle ground would be confinement to your cell with a book or two, but still in the presence of people. You'd take your meals there, not in the cafeteria. You wouldn't get to hang out with people or go outside. The jail equivalent of a time out. Not fun, probably boring, but not literal psychological torture.

Prison already is psychological torture.

Perhaps not in (for example) the Nordic model. But in some primitive countries, methods to mentally and emotionally torture prisoners seem to have become some sort of art form.

They’re already in prison, applying extra pain with no trial or anything like that is not very civilized, to the contrary.

Ban visitors?