CCC 1583 It is true that someone validly ordained can, for grave reasons, 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,1 because the character imprinted by ordination is for ever. The vocation and mission received on the day of his ordination mark him permanently.
1 Cf. CIC, cann. 290-293; 1336 § 1 3o, 5o, 1338 § 2; Council of Trent: DS 1774.