Objection 1. It would seem that the Church ought not to excommunicate anyone, because excommunication is a kind of curse, and we are forbidden to curse (Romans 12:14). Therefore the Church should not excommunicate.
Objection 2. Further, the Church Militant should imitate the Church Triumphant. Now we read in the epistle of Jude (verse 9) that "when Michael the Archangel disputing with the devil contended about the body of Moses, he durst not bring against him the judgment of railing speech, but said: The Lord command thee." Therefore the Church Militant ought not to judge any man by cursing or excommunicating him.
Objection 3. Further, no man should be given into the hands of his enemies, unless there be no hope for him. Now by excommunication a man is given into the hands of Satan, as is clear from 1 Corinthians 5:5. Since then we should never give up hope about anyone in this life, the Church should not excommunicate anyone.
On the contrary, The Apostle (1 Corinthians 5:5) ordered a man to be excommunicated.
Further, it is written (Matthew 18:17) about the man who refuses to hear the Church: "Let him be to thee as the heathen or publican." But heathens are outside the Church. Therefore they also who refuse to hear the Church, should be banished from the Church by excommunication.
I answer that, The judgment of the Church should be conformed to the judgment of God. Now God punishes the sinner in many ways, in order to draw him to good, either by chastising him with stripes, or by leaving him to himself so that being deprived of those helps whereby he was kept out of evil, he may acknowledge his weakness, and humbly return to God Whom he had abandoned in his pride. In both these respects the Church by passing sentence of excommunication imitates the judgment of God. For by severing a man from the communion of the faithful that he may blush with shame, she imitates the judgment whereby God chastises man with stripes; and by depriving him of prayers and other spiritual things, she imitates the judgment of God in leaving man to himself, in order that by humility he may learn to know himself and return to God.
Reply to Objection 1. A curse may be pronounced in two ways: first, so that the intention of the one who curses is fixed on the evil which he invokes or pronounces, and cursing in this sense is altogether forbidden. Secondly, so that the evil which a man invokes in cursing is intended for the good of the one who is cursed, and thus cursing is sometimes lawful and salutary: thus a physician makes a sick man undergo pain, by cutting him, for instance, in order to deliver him from his sickness.
Reply to Objection 2. The devil cannot be brought to repentance, wherefore the pain of excommunication cannot do him any good.
Reply to Objection 3. From the very fact that a man is deprived of the prayers of the Church, he incurs a triple loss, corresponding to the three things which a man acquires through the Church's prayers. For they bring an increase of grace to those who have it, or merit grace for those who have it not; and in this respect the Master of the Sentences says (Sent. iv, D, 18): "The grace of God is taken away by excommunication." They also prove a safeguard of virtue; and in this respect he says that "protection is taken away," not that the excommunicated person is withdrawn altogether from God's providence, but that he is excluded from that protection with which He watches over the children of the Church in a more special way. Moreover, they are useful as a defense against the enemy, and in this respect he says that "the devil receives greater power of assaulting the excommunicated person, both spiritually and corporally." Hence in the early Church, when men had to be enticed to the faith by means of outward signs (thus the gift of the Holy Ghost was shown openly by a visible sign), so too excommunication was evidenced by a person being troubled in his body by the devil. Nor is it unreasonable that one, for whom there is still hope, be given over to the enemy, for he is surrendered, not unto damnation, but unto correction, since the Church has the power to rescue him from the hands of the enemy, whenever he is willing.
Source: Summa Theologica (New Advent)