The term excommunicate is interchangeable with banishment as used by many. Even if he is a bishop, "Bishop" Amobrose cannot excommunicate anyone without jurisdiction. A priest cannot excommunicate anyone. Members of the laity cannot excommunicate. The worst any of these can do is "vote you off the island." Unless a person is prevented by civil law from entering a particular chapel, there is technically nothing that can be done if the facility is tax-exempt. When it comes to chapels that meet on private property, all bets are off. The owner of the property has a civil and juridical right to bar a person from his property. ie. Nobody has a right to enter the land of OLMC.
Even if a chapel is tax exempt, the priest and laity can make it nearly impossible for an unwanted person to hear Mass. (Think, harassment on line, damage to vehicle, graffiti on vehicle, harassment on the street outside one's home...) The priest can refuse absolution, communion, any and all sacraments. One can be told directly by the ushers to leave. If the victim returns with the police, the guilty parties will deny everything. The outward effects of continual harassment are basically the same as being excommunicated, but in God's eyes, there is no excommunication.
Just wanting to get this right. Those who continually claim to have been excommunicated, or that Bp. or Fr. so-and-so excommunicated someone is nonsense.