What is the ruling on masturbation if it is for the purpose of expelling excess semen to avoid nocturnal emissions, knowing that no women are imagined during its practice?

You must repent to Allah Almighty and seek forgiveness for the sin of masturbation by abandoning it, regretting what has passed, and resolving to abstain from it in the future. Among the most important means to help with this is marriage. If you are unable to marry, then you must fast, engage in strenuous activities, and keep righteous company. As for a wet dream (احتلام), there is no blame for it.