David Bradshaw's paper "Patristic Views on Why There Is No Repentance after Death" is very interesting. He explains how St. Maximus follows St. Dionysius the Areopagite in teaching that all human beings have the free will to either choose God or reject Him, and their eternal fate will simply be the perpetual realization of this free choice. Those who choose God, who is the very principle of goodness and being itself, will themselves naturally tend towards higher states of goodness and being, which finds its fullest culmination in the Age to Come when God is all-in-all. When this happens, there will be no more options to turn away from God, He will simply be fully-filling all things, and so those who have disposed themselves towards receiving God in this life will eternally experience the fullness of the divine presence as a state of never-ending bliss and happiness.
For those who did not choose God in this life, however, it's a different story. It's important to understand that for St. Maximus, when we choose to sin, we are in fact diminishing our own free will. This is sometimes referred to as "the paradox of freedom," which is that the more you freely choose to do whatever you want, as opposed to what God wants (which is what many people think freedom is), the more you're actually destroying your ability to exercise free choice. The classic example of this is addiction. The more you choose to participate in vices like drugs or pornography, the harder and harder it will be for you to break free of these grave sins, because the sins themselves destroy your freedom.
With that in mind, consider St. Maximus' description of the Age to Come. In this Age, all souls, including the damned, will be incapable of moving towards anything that isn't God since He will be all-in-all. For the righteous, this is bliss because their true desire is simply God Himself and so they are able to eternally move closer and closer to Him whom they long for. However for the wicked, this is torment because they have oriented their wills such that they only desire things other than God, and indeed are incapable of desiring God Himself because of their choice to destroy their own freedom; but because the only "thing" anyone can move towards in the Age to Come is God, and the wicked have rendered themselves incapable of doing that, their only option is to move towards the evil they truly desire, which is simply non-existence.
And it's this eternal movement of the damned towards non-being that constitutes their torment. Just as the blessed become more and more real as their participation in God eternally grows, so too will the damned become less and less real as they perpetually turn away from Him. They will never be totally annihilated, because God’s love for them will always sustain their existence, however they’ll get about as close to that fate as is metaphysically possible. As C.S. Lewis puts it in The Great Divorce:
All Hell is smaller than one pebble of your earthly world: but it is smaller than one atom of this world, the Real World. Look at yon butterfly. If it swallowed all Hell, Hell would not be big enough to do it any harm or to have any taste' 'It seems big enough when you're in it, Sir.' 'And yet all loneliness, angers, hatreds, envies, and itchings that it contains, if rolled into one single experience and put into the scale against the least moment of the joy that is felt by the least in Heaven, would have no weight that could be registered at all. Bad cannot succeed even in being bad as truly as good is good. If all Hell's miseries together entered the consciousness of yon wee yellow bird on the bough there, they would be swallowed up without trace, as if one drop of ink had been dropped into that Great Ocean to which your terrestrial Pacific is only a molecule' 'I see,' said I at last. 'She couldn't fit into Hell.'