The indelible character
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. The vocation and mission received on the day of his ordination mark him permanently.
Source: Catechism of the Catholic Church (Vatican.va)