What should I do about someone who curses Allah or the Prophet (peace be upon him) in front of me?

Whoever reviles Allah or His Messenger (peace be upon him) has disbelieved and exited the fold of Islam, and he must be deterred by every possible means. If there is an authority that can prevent and punish him, then his case should be referred to it. Otherwise, the obligation is to denounce him according to one's capacity. However, the legal punishment (hudud) can only be administered by the ruler (Sultan). It is not permissible to sit with him while he is committing this evil, if you are unable to forbid him, for Allah Almighty says: "And He has already sent down 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" (An-Nisa: 140).