Musar על ערכין 30:17

Torah study is also a preventative against one's coming to (speak) lashon hara, as they said there (Arachin 15b): "What is the preventative against lashon hara? If he is a Torah scholar, let him occupy himself with Torah." The reason is obvious, as we find in Sanhedrin 99b: "All men are created for toil, viz. (Iyyov 5:7): 'For man is born for toil.' I would not know whether for toil of the lips or toil of labor — if it were not written: (Mishlei 16:22): 'For he saddles his mouth with it,' which indicates that he was born for toil of the lips. And I still would not know whether for the [lip] toil of Torah or the toil of converse — if it were not written (Joshua 1:8): 'Let the book of this Torah not depart from your mouth,' which indicates that he was created for the toil of Torah." The idea is that the nature was given man, when speech was given to him, that the tools of speech do what is theirs without becoming fatigued, unlike the other organs, which man must spur himself to put into operation. Not so, the faculty of speech. And this is the intent of "the toil of the lips." But a man must reflect that this power was given him only to merit Torah. Therefore, it is stated that the preventative of speaking lashon hara is only occupying oneself with Torah, for lacking this, he will certainly stumble into forbidden speech. For it is man's nature that his tools of speech not be inoperative (unless he is able to overcome his yetzer and make himself "mute," as they say in Chullin 89a).

See the aptness of these words of Chazal, who compared one afflicted by this sin to one bitten by a mad dog. For, in truth, both are alike. As stated in Yoma 84a: "One bitten by a mad dog has no cure." And about lashon hara they have also said that there is no remedy for one who is habituated to this sin, G-d forbid, viz. (Arachin 15b): "If he has already slandered, there is no remedy for him."