What is the easiest way to definitively stop masturbating for someone who has fallen into it and repented from it repeatedly?

Masturbation is forbidden (haram). The way to overcome it is by adhering to prayer (du'a), learning Islamic knowledge, associating with righteous people, occupying free time with beneficial activities, staying away from causes of temptation and incitements to lust, and observing frequent fasting. Do not despair if you stumble; rather, hasten to sincere repentance.