According to Spiritism, there is no place, or condition, of eternal punishment. In fact, God does not truly "punish". Instead, his laws allow for reparation and redemption, along with learning and progressive growth, through the processes of reincarnation. Whenever we violate the Law of Love and cause suffering to befall upon others, our spirit is blemished with the marks of the transgression and our conscience is left in unrest. Through Spiritism, however, we learn that rather than condemning his children eternally for their mistakes, God gives us, through reincarnation, limitless opportunities to purify our souls, to re-harmonize ourselves with one another, and to heal from the painful consequences of wrongdoing.
Below is are excerpts from Allan Kardec's book "What is Spiritism?"1, from the section including a "mock" dialogue between himself and a priest. In this excerpt, he is answering a comment about religious/philosophic principles in which Spiritism differs from the Church:
In the second part of his answer, Kardec wrote:
"Here's what the Spiritist Doctrine says in this regard: The duration of punishment is subordinate to the rehabilitation of the guilty spirit. No predetermined amount of time is pronounced for punishment of the spirit. What the Law requires in order to terminate the spirit's suffering is repentance and reparation - in short, a serious and effective improvement, a sincere return to good. The spirit, thus, controls its own destiny. Its obstinacy in wrongdoing prolongs its suffering; likewise, its efforts to do good lessen or abbreviate its suffering. With the length of suffering dependent upon a change of attitude, the guilty spirit that doesn't repent and never improves, suffers, so to say, forever. For that consciousness, then, there is an eternity of punishment. This suffering must be understood in the relative sense and not in the absolute. A condition inherent to the inferiority of certain spirits is that they can't see an end to their situation, and they believe they must suffer forever. But when their souls open to repentance, God permits them to glimpse a ray of hope.
Certainly, this principle confirms more to the justice of God than the Church's dogma of eternal punishment. According to this view, Providence disciplines while the guilty one persists in wrongdoing, and bestows grace when the guilty one returns to the path of good. Who could have imagined that theory? Could we have been the ones? No. The spirits teach it and give evidence supporting it through the examples they provide to us daily. The spirits, then, don't deny future punishment; in fact, they come to describe their own suffering. This portrayal is more convincing than the thought of perpetual flames, because everything in it is perfectly logical. The spirits' examples make it plainly clear that this situation is a natural consequence of things. Even the inquisitive philosopher is better able to accept it, because nothing in it contradicts reason. This is why Spiritist beliefs have guided many people toward good, even materialists, upon whom the fear of hell made no impression."
In the first part of his answer, immediately before the text above, Kardec wrote:
"Believe in the flames and tortures, if you wish, as long as your belief impedes the practice of harmful deeds. However your faith won't make these things real if they don't exist. You believe that we have only one corporeal existence, but your belief won't prevent spirits from being reborn here or in some other place, if it must be, despite your wish to the contrary. You believe the world was created in six days of twenty-four hours each but, in spite of that, the Earth itself presents contradictory proof in its geological layers. You are convinced that Joshua made the sun stop, which doesn't change the fact that it's the Earth that rotates. You say that the date of mankind's first appearance on Earth was not more than 6,000 years ago. This doesn't prevent the facts from contradicting this idea, though. And what will be the reply when geologists prove, through obvious vestiges, that mankind's ancestors predate that period, just as they have already proven so many other things?
Believe, then, in anything you wish - even in the existence of the devil - if such a belief will make you a better person and more charitable toward your fellow man. The Spiritist Doctrine, as an ethical philosophy, recommends only one thing: the necessity of doing good and avoiding doing wrong. It is a science of observation that, I repeat, has ethical consequences that confirm and prove those great religious principles. As far as other secondary points, it leaves them at the discretion of each.
[...] If you had read everything we've written on this subject, you would have seen that the Spiritist Doctrine limits itself to giving such religious controversies a more logical and rational interpretation than the one commonly provided by the Church. The Spiritist Doctrine does not, for example, deny purgatory; rather it demonstrates the need for, and justice of, the concept of purgatory. The Church has described hell as an immense furnace, but is it understood that way by higher theology? Evidently not. Theologians contend that this is simply imagery, that the fire is imaginary, a symbol for greater suffering. And if it were possible to know the intimate thoughts of all humanity - even the most religious - who can reason and understand the eternity of punishment, you would find that the majority believe that the idea of an eternity of suffering is a denial of the infinite mercy of God."
1 Text taken from the book, "What is Spiritism", first published in France in the late 1860s, by Allan Kardec, as translated and published, in 2004, by the Allan Kardec Education Society, in the book "Introduction to the Spiritist Philosophy", a combination of two of Allan Kardec’s works: Qu’Est-Ce Que Le Spiritisme? (What is Spiritism?) and Spiritisme Reduit a sa plus Simple Expression (Spiritism in its Simplest Expression).