Wikipedia
A vitandus ( Latin for "one to be avoided") was someone affected by a rare and grave form of excommunication, in which the Church ordered, as a remedial measure, that the faithful were not to associate with him "except in the case of husband and wife, parents, children, servants, subjects", and in general unless there was some reasonable excusing cause.
It thus imposed a form of shunning somewhat similar to Jewish cherem.
Since the coming into effect of the 1983 Code of Canon Law, this form of excommunication is no longer envisaged in the canon law of the Catholic Church. The 1917 Code still included it, imposing it automatically (a latae sententiae excommunication) on anyone who did physical violence to the Pope himself, and declaring that with that exception, "nobody is a vitandus excommunicate unless the Apostolic See has excommunicated him by name and has proclaimed the excommunication publicly and in the decree has stated expressly that he must be avoided".
The distinction between a vitandus and a toleratus ("tolerated") excommunicate was introduced by Pope Martin V in 1418.
The most notable case in the 20th century of excommunication with the effect of making the person a vitandus was that of the priest Alfred Loisy.
In 1950, antipope Michel Collin, of the sedevacantist group Apostles of Infinite Love, announced that he had taken the name Clement XV. Pope Pius XII laicized him in 1951 and publicly declared him by name a vitandus excommunicate.