
Do you have a reference for that? Excommunication, as I understand it, is a formal process. From the Catholic Encyclopedia
newadvent.org/cathen/05678a.htm :
Who can excommunicate?
Excommunication is an act of ecclesiastical jurisdiction, the rules of which it follows. Hence the general principle: whoever has jurisdiction in the forum externum, properly so called, can excommunicate, but only his own subjects. Therefore, whether excommunications be a jure (by the law) or ab homine (under form of sentence or precept), they may come from the pope alone or a general council for the entire Church; from the provincial council for an ecclesiastical province; from the bishop for his diocese; from the prelate nullius for quasi-diocesan territories; and from regular prelates for religious orders. Moreover, anyone can excommunicate who, by virtue of his office, even when delegated, has contentious jurisdiction in the forum externum; for instance, papal legates, vicars capitular, and vicars-general. But a parish priest cannot inflict this penalty nor even declare that it is incurred, i.e. he cannot do so in an official and judicial manner. The subjects of these various authorities are those who come under their jurisdiction chiefly on account of domicile or quasi-domicile in their territory; then by reason of the offence committed while on such territory; and finally by reason of personal right, as in the case of regulars.