The Catholic church doesn't kick people out with an excommunication.
definition: to sentence (a member of the Church) to exclusion from the communion of believers and from the privileges and public prayers of the Church
If a priest gets excommunicated they don't suddenly stop being a catholic priest. They would still preach Christianity. Sure they couldn't go to rome and kiss the popes ring, but they don't stop being a priest. The rogue SA priest you speak of is essentially excommunicated. Any Theocratic realm he ends up in is going to ban him. He will be ignored by most when speaking in the church.
