What is the ruling on thinking about forbidden things—such as fornication—without committing them, and how can one get rid of their effects?

So long as thoughts are mere fleeting notions that have not been acted upon or insisted upon, there is no accountability for them. Rather, one is rewarded for repelling them. Seek refuge with Allah from Satan when they recur, and direct your heart to what benefits you in your religion and your worldly life. Be diligent in keeping company with the righteous and hasten to marry. If you are unable to marry, then you must fast, as the Prophet, peace and blessings be upon him, said: "O young people, whoever among you can afford to marry, let him do so, for it is more apt to lower the gaze and guard the private parts. And whoever is unable to do so, let him fast, for it will be a shield for him."