The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
41. And not also (can the expiation take place) prescribed in the chapter treating of qualification, because on account of the inference of his lapse from Smriti he (the Naishthika) is not capable of it.
If a Brahmakârin for life breaks from inattention the vow of chastity, is he to perform the expiatory sacrifice enjoined by the text, 'A student who has broken the vow of chastity shall sacrifice an ass to Nirriti 1' or not?--He is not, the pûrvapakshin says. For although in the chapter which treats of qualification (Pû. Mîm. Sû. VI, 8, 22) that expiatory ceremony has been settled (for Brahmakârins in general), it does not yet hold good for the professed Brahmakârin. For Smriti declares that such sins cannot be expiated by him any more than a head once cut off can again be healed on to the body, 'He who having once entered on the duties of a Naishthika again lapses from them, for him--a slayer of the Self--I see no expiation which might make him clean again. 'The Upakurvâna (i.e. he who is a Brahmakârin for a certain time only, not for life) on the other hand, about whose sin Smriti makes no similar declaration, may purify himself by the ceremony mentioned.