Yeah, no shade to you at all. I use the term ethics in such a broad way (c.f. my conversation with Vitor) that it's really not possible to have an ethic beyond reproach.
For this one, last word has the same problem block does. You can't make someone else stop talking, you can only stop listening. The ethical argument is that if you're the kind of person who forces the other person to stop talking rather than removing yourself from the conversation, you're relying on clients to exercise force on your behalf. Sometimes that's ok, Rabble has convinced me that blocks and deletes are worth supporting to make online spaces easier for women (for example) to be in. So it's not a hard and fast opinion, I just would feel like a jerk if I ever used last word, and would probably be percieved as such.