If you are an atheist who was once a Catholic, you should consider having yourself excommunicated. There are few more definitive and public steps you can take on behalf of the rejection of religion. Unless and until you are excommunicated, you are still counted as a Catholic. Why would you allow them to regard you as even provisionally Catholic? What are you afraid of? Do you still have doubts about believing in God and are keeping your options open in case you want to return to the Church?