Nedarim 7 - I am excommunicated from you!
Having discussed vague declarations of vows for charity (corner of the field), and an equally vague designation of a lavatory - and left without an answer to both - the Talmud comes to another rule, about a man to told his fellow, "I am excommunicated from you!"
Rabbi Akiva struggled with this case, and even though he might rule stringently and maintain that this creates a vow, he still remained in doubt. This meant that there would be no punishment for violating of such vow. Most of the times, when people use the word "excommunicate," they mean a religious disciplinary measure, not a vow to abstain from dealing with one's fellow.
Incidentally, a Sage who hears bad words from his fellow should excommunicate him. But it is not nice for a Sage to excommunicate anybody, so what is he to do? - He should then annul his excommunication. Or, he can be like Mar Zutra who, when he needed to excommunicate one of his students, would first excommunicate himself, and then the student. On coming home, he would annul excommunication for himself, and then for the student.
Art: Before vows by Vasily Perov