"Destruction...means the unmaking, or cessation, of the destroyed. And people often talk as if the 'annihilation' of a soul were intrinsically possible. In all our experience...the destruction of one thing means the emergence of something else. Burn a log, and you have gases, heat and ash. To have been a log means now being those three things. If soul can be destroyed, must there not be a state of having been a human soul? And is not that, perhaps, the state which is equally well described as torment, destruction, and privation? You will remember that in the parable, the saved go to a place prepared for them, while the damned go to a place never made for men at all. To enter heaven is to become more human than you ever succeeded in being in earth; to enter hell, is to be banished from humanity. What is cast (or casts itself) into hell is not a man: it is 'remains.'" (The Problem of Pain)
Lewis' insight here also helps us to understand how the saints in Heaven can (and do) enjoy their blessedness even though they know that the unrepentant are in Hell. The damned simply are no longer fully human, no longer human in a truly meaningful sense of the word. They are "remains." These remains, although they do indeed suffer, cannot spoil the joys of Heaven for the saved.