Bava Metzia 59b says that at the end of the episode of the Oven of Akhnai, Rabbi Eliezer ben Hyrcanus was excommunicated. (Apparently there's a makhloket about whether this was nidui or cherem.) The ...
Let's suppose that someone in a community has been put in cheirem, for whatever reason. This would generally ban him from the community shul. Should this person show up in the community shul, however, ...