Gemara

(Rav Acha bar Chanina): If he already spoke Lashon ha'Ra, he cannot correct this. He already received Kares - "Yachres Hash-m Kol Sifsei Chalakos Lashon Medaberes Gedolos"!

Yoma 85b (Mishnah): Yom Kipur atones for Aveiros Bein Adam la'Makom. Yom Kipur does not atone for Aveiros Bein Adam l'Chavero until he appeases the victim.

87a (R. Yitzchak): One who provoked his friend, even verbally, must appease him. It says "Nokashta v'Imrei Ficha... Lech Hisrapes u'Rhav Re'echa." (If you spoke improperly, humble yourself in front of him, and put him over yourself.)

Rishonim

Rambam (Hilchos De'os 7:2): A Rachil is one who goes around saying 'Ploni said such and such, I heard such and such about Ploni.' He destroys the world, even if it is true. Worse than this is Lashon ha'Ra, which is seeking detriment about another, even if it is true. A Ba'al Lashon ha'Ra is one who sits and says "Shimon did so and so, and his ancestors did so and so, and I heard this about him", and says derogatory things. It says about this "Yachres Hash-m Kol Sifsei Chalakos Lashon Medaberes Gedolos"!

Sha'arei Teshuvah (of R. Yonah, 3:204): Teshuvah for a Ba'al Lashon ha'Ra is very hard. Since he accustomed himself to speak Sheker and evil, he does not rule over his spirit. His lips are not under his control.

Sha'arei Teshuvah (207): If a Ba'al Lashon ha'Ra repents, he must ask pardon from those he spoke about. He will not remember all of them. He wounded many, and they do not know that he caused evil for them. He will be ashamed to tell them that he did evil to them, for he wounds anonymously. This is why Lashon ha'Ra is compared to arrows. Often, the victim does not know who hit him.

Be'er Mayim Chayim (49): This is why the Gemara says that there is no remedy for Lashon ha'Ra. Seemingly, nothing blocks Teshuvah! Even if Chas v'Shalom someone was Kofer b'Ikar (denied basic tenets of Judaism), Teshuvah helps for this, like the Rambam says (Hilchos Teshuvah 3:4). How can the Gemara say that there is no remedy?! According to R. Yonah, we can say that the Gemara discusses one who wantonly transgresses Lashon ha'Ra. The verse discusses this, like the Rambam says in Hilchos De'os. Teshuvah helps only after appeasing the victim, and this is almost impossible. This is like the Gemara (Yevamos 21a) says about false measures, that one cannot do Teshuvah for this (because he does not remember everyone from whom he stole).

Sha'arei Teshuvah (208): Also, if one pulled out a sword (with intent to kill) and his mercy was aroused, he can return it to its sheath. One who shot an arrow cannot take it back. Similarly, a Ba'al Lashon ha'Ra cannot fix words after they left his mouth. Sometimes one speaks about a blemish in (lineage of) a family, and damages all coming generations. He will not get forgiveness for this. The Yerushalmi (Bava Kama Sof Perek 8) says that he will never get atonement for it. One who sends his tongue (freely speaks Lashon ha'Ra) will also speak about living Kedoshim (great Tzadikim, which is Apikorsus). An Apikoros has no share in the world to come (Sanhedrin 90a).

Acharonim

Chafetz Chaim (4:12): If Ploni spoke Lashon ha'Ra and wants to repent, it depends on the reaction:

If the listeners rejected his words, and the subject (Reuven) was not disgraced in their eyes at all, Ploni's sin is only between him and Hash-m. He transgressed Hash-m's commanded not to speak Lashon ha'Ra. To fix this, he must regret the past, confess and accepts with a full heart for the future not to do so, like any Aveirah between man and Hash-m.

If Reuven was disgraced in their eyes, and this caused damage to Reuven's body or property, or pained him, this is a sin Bein Adam l'Chavero. Even Yom Kipur and death do not atone until he appeases Reuven. Therefore, he must request pardon from Reuven. When he appeases him and Reuven forgives him, there remains only a sin between Ploni and Hash-m. He fixes it like we said above.

Be'er Mayim Chayim (47): Even one who verbally provoked someone must appease him. All the more so, this applies if Reuven was lowered in the eyes of the listeners, and this pained him! If Reuven is not appeased, one must do like it says in OC 606:1-2 (he goes three times to appease him, and brings friends to speak to him...)

Chafetz Chaim (ibid.): Even if Ploni does not know about this (the Lashon ha'Ra), Ploni must tell him what he did improperly to him and request pardon, since Ploni knows that he caused it. This shows how much one must avoid this lowly Midah, for one who is habituated to it, Chas v'Shalom, it is almost impossible for him to repent.

Be'er Mayim Chayim (48): If no damage came to Reuven yet, but with time damage or pain might comes to him, must Ploni appease him? Surely, if Ploni repented only to Hash-m, and later damage came to Reuven due to Ploni's words, he must appease him. The question is whether Reuven must appease him before damage occurs. This requires investigation. It is best to cause that no damage will come to him. E.g. he goes to the listener and says 'I erred in what I said about Reuven. Based on what I see now, the matter was not like I said...' or a similar retraction, until the listener will remove the matter from his mind.