Prisons

All the problems that you hear with prisons like killings and bad influences. Isnt the simple answer be just have all prisoners be in isolation. Don't let them mingle at all. Have them stay in their cells indefinitely.

OP obviously has no idea about corrections.  Keeping prisoners isolated from one another would not only be extremely difficult but also dangerous.  Humans without proper socialization become extremely violent when confronted even in a non threatening way.  Keeping the prisoners in their cells all day would increase violence and suicide.

Remember, the goal of corrections is to rehabilitate the prisoner not isolate them!