Yes, I am aware of the fact that some people are adamant that the word "kushi" is harmless and wonderful, and that black people shouldn't feel hurt when this word is used to disparage them.[1]
Rather than trying to convince people that certain words are always ok or not ok to use, you should give reasons for others to reconsider their usage of the word and prompt them to think of the context before using the word. Then you let them make the choice of when to use it.