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.
Actually, yes, they do. They could call themselves a priest, but legally they would not be, and they would not be a member of the Catholic (big C, is an important difference) Church, and thus would not be a Catholic priest even if they started their own religion or joined another one . . . they'd be some other sort of priest.
The Catholic Church, along with other churches, also has a procedure known as defrocking, which does not excommunicate someone, but simply removes them from being a priest (or other ordained ministry).
