What should I do if I hear someone cursing and reviling God, and I have forbidden him from doing so, but he insists on continuing his actions?

Reviling Allah Almighty is among the gravest forms of disbelief, and it is one of the nullifiers of Islam by scholarly consensus. The implementation of the apostate's punishment is exclusive to the ruler, not to individuals. Your duty is to denounce the perpetrator in whatever way you can. It is also imperative to separate from him and not sit with him while he persists in this crime, if he does not desist from it; based on Allah’s Almighty saying: "And He has already revealed to you in the Book that when you hear the verses of Allah being denied and ridiculed, do not sit with them until they enter into another conversation. Indeed, in that case, you would be like them."