Catechism of the Catholic Church §1583 — excerpt
It is true that someone validly ordained can, for a just reason, be discharged from the obligations and functions linked to ordination, or can be forbidden to exercise them; but he cannot become a layman again in the strict sense, because the character imprinted by ordination is for ever.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: CIC, cann. 290-293 · 1336 # 1 3, 5, 1338 # 2 · Council of Trent DS 1774