This passage may be analyzed by pulling apart the phrases in it.
[the shall be as though there had been no redemption made] It must first be remembered that this verse deals with one who is unrepentant and is judged of God. This phrase indicates that for the unrepentant, they are in the same condition as if Christ had never atoned for them. In this specific case, Alma is speaking of the redemption from sin, not the redeption from death. To understand Alma's argument, we must understand that he separates the atonement into two separate aspects, the atonement for death and the atonement for sin. This argument depends upon the completion of the atonement for death (the person is resurrected) but the personal denial of the atonement for sin (the person remains sinful).
[for they cannot be redeemed according to God's justice] The reason that this person is unaffected by the atonement is that it is impossible for God to redeem him. The atonement for sin is conditional, and the person has not met the conditions. Thus a just God is unable to redeem him. This is not God's choice, it is the choice of the one who elected to remain unrepentant, and therefore sinful. Having rejected the way God provided for the removal of sin, the person is now without any other recourse. Not even the all powerful God can help him, for God is just, and justice requires that the law be applied.
[and they cannot die] This is where we must remember that Alma has separated the aspects of the atonement. This is a person who has been resurrected, therefore, he cannot die. Without death to change one's state, there is no opportunity to come for future repentance.
[seeing there is no more corruption] This phrase depends upon the language of Paul (see, for instance, 1 Corinthians 15:42-50). The "corruption" is mortal life. The phrase might be rewritten "seeing there is no more mortality." Since mortal life is over, and the resurrection has supplied a body that will not die, the person is condemned forever with no means of changing his fate.