By collating similar passages with 1 Samuel 3:25 , St. Ambrose shows that the meaning is not that no one shall intercede, but that the intercessor must be worthy as were Moses and Jeremiah, at whose prayers we read that God spared Israel.
40. But you say, It is written: “If a man sin against the Lord, who shall entreat for him?” First of all, as I already said before, I might allow you to make that objection if you refused penance to those only who denied the faith. But what difficulty does that question produce? For it is not written, “No one shall entreat for him;” but, “Who shall entreat?” that is to say, the question is, Who in such a case can entreat? The entreaty is not excluded.
41. Then you have in the fifteenth Psalm: “Lord, who shall dwell in Your tabernacle, or who shall rest upon Your holy hill?” It is not that no one, but that he who is approved shall dwell there, nor does it say that no one shall rest, but he who is chosen shall rest. And that you may know that this is true, it is said not much later in the twenty-fourth Psalm: “Who shall ascend into the hill of the Lord, or who shall stand in His holy place?” The writer implies, not any ordinary person, or one of the common sort, but only a man of excellent life and of singular merit. And that we may understand that when the question is asked, Who? It does not imply no one, but some special one is meant, after having said “Who shall ascend into the hill of the Lord?” the Psalmist adds: “He that has clean hands and a pure heart, who has not lift up his mind unto vanity.” And elsewhere it is said: “Who is wise and he shall understand these things?” And in the Gospel: “Who is the faithful and wise steward, whom the Lord shall set over His household to give them their measure of wheat in due season?” And that we may understand that He speaks of such as really exist, the Lord added: “Blessed is that servant, whom his Lord when He comes shall find so doing.” And I am of opinion that where it is said, “Lord, who is like You?” it is not meant that none is like, for the Son is the image of the Father.
42. We must then understand in the same manner, “Who shall entreat for him?” as implying: It must be some one of excellent life who shall entreat for him who has sinned against the Lord. The greater the sin, the more worthy must be the prayers that are sought. For it was not any one of the common people who prayed for the Jewish people, but Moses, when forgetful of their covenant they worshipped the head of the calf. Was Moses wrong? Certainly he was not wrong in praying, who both merited and obtained that for which he asked. For what should such love not obtain as that of his when he offered himself for the people and said: “And now, if You will forgive their sin, forgive; but if not, blot me out of the book of life.” We see that he does not think of himself, like a man full of fancies and scruples, whether he may incur the risk of some offense, as Novatian says he dreads that he might, but rather, thinking of all and forgetful of himself, he was not afraid lest he should offend, so that he might rescue and free the people from danger of offense.
43. Rightly, then, is it said: “Who shall entreat for him?” It implies that it must be such an one as Moses to offer himself for those who sin, or such as Jeremiah, who, though the Lord said to him, “Pray not for this people,” and yet he prayed and obtained their forgiveness. For at the intercession of the prophet, and the entreaty of so great a seer, the Lord was moved and said to Jerusalem, which had meanwhile repented for its sins, and had said: “O Almighty Lord God of Israel, the soul in anguish, and the troubled spirit cries unto You, hear, O Lord, and have mercy.” And the Lord bids them lay aside the garments of mourning, and to cease the groanings of repentance, saying: “Put off, O Jerusalem, the garment of your mourning and affliction. and clothe yourself in beauty, the glory which God has given you for ever.”
Source: On Repentance (New Advent)