Is it permissible to rebuke one who tampers with words of disbelief, such as changing letters in words like "يلعن رئك" (may your opinion be cursed), "يلعن الص" (may the 'saad' be cursed), and "يلعن رفك" (may your companion be cursed)?

You are right to reprimand them, for their utterance is close to disbelief (kufr), and changing a letter does not remove the forbidden aspect. The religion has a sacred boundary that must be protected. Approaching disbelief has degrees, and a Muslim is not permitted to approach any of them; rather, the obligation is to keep completely away, because such words trivialize disbelief in people's souls. Furthermore, changing a letter does not nullify the effect of the word, as the altered word is a symbol or an indication of disbelief. A Muslim must guard their tongue, for perhaps a word they utter, which they do not consider significant, becomes the cause of their destruction. The Prophet, peace and blessings be upon him, said: "Indeed, a servant may utter a word, without thinking much of it, by which he will fall into the Fire further than the distance between the East." And cursing in itself is a major sin, and this alleged masculinity gained through cursing and swearing is false and invalid.