Objection 1. It would seem that sentence of excommunication can be passed on a body of men. Because it is possible for a number of people to be united together in wickedness. Now when a man is obstinate in his wickedness he should be excommunicated. Therefore a body of men can be excommunicated.
Objection 2. Further, the most grievous effect of an excommunication is privation of the sacraments of the Church. But sometimes a whole country is laid under an interdict. Therefore a body of people can be excommunicated.
On the contrary, A gloss of Augustine [Cf. Ep. ccl] on Matthew 12 asserts that the sovereign and a body of people cannot be excommunicated.
I answer that, No man should be excommunicated except for a mortal sin. Now sin consists in an act: and acts do not belong to communities, but, generally speaking, to individuals. Wherefore individual members of a community can be excommunicated, but not the community itself. And although sometimes an act belongs to a whole multitude, as when many draw a boat, which none of them could draw by himself, yet it is not probable that a community would so wholly consent to evil that there would be no dissentients. Now God, Who judges all the earth, does not condemn the just with the wicked (Genesis 18:25). Therefore the Church, who should imitate the judgments of God, prudently decided that a community should not be excommunicated, lest the wheat be uprooted together with the tares and cockle.
The Reply to the First Objection is evident from what has been said.
Reply to Objection 2. Suspension is not so great a punishment as excommunication, since those who are suspended are not deprived of the prayers of the Church, as the excommunicated are. Wherefore a man can be suspended without having committed a sin himself, just as a whole kingdom is laid under an interdict on account of the king's crime. Hence there is no comparison between excommunication and suspension.
Source: Summa Theologica (New Advent)