§1. The following are effected to vote: 1/ a person incapable of a human act; 2/ a person who lacks active voice; 3/ a person under a penalty of excommunication whether through a judicial sentence or through a decree by which a penalty is imposed or declared; 4/ a person who has defected notoriously from the communion of the Church.
§2. If one of the above is admitted, the person’s vote is null, but the election is valid unless it is evident that, with that vote subtracted, the one elected did not receive the required number of votes.
Source: Code of Canon Law (Vatican.va)