What is the ruling on someone who swears by Almighty Allah to abandon a sin (such as smoking or drinking alcohol), then breaks his oath and commits the sin, and pays the expiation, and this is repeated by him many times?

For every oath you have broken, you must offer expiation. Abandoning sin is not achieved by swearing an oath to abandon it, especially when one frequently returns to it. This is because it leads to excessive swearing, which is prohibited by Allah's saying: "And do not make [your oath by] Allah an excuse against being righteous and fearing Allah and making peace among people. And Allah is Hearing and Knowing" (Al-Baqarah: 224). The best way to abandon sins is through sincere and truthful repentance, with its three conditions: abandoning the sin, regretting it, and having a firm resolve not to return to it. If you return to the sin, do not despair, but renew your repentance, for Allah accepts the repeated repentance of His servant.