It's NEVER a good idea to "play" with grenades, and "N" is a grenade waiting to go off. 1st mention of it, player should be advised, "I (you) don't want to hear that word again, if you have a problem give me a number. Use it again, and you're done for the day".
If that player is dumb enough to test you, and use it again, HE'S GONE (no excuses, no exceptions).
If that word is repeated on the field, and blows up, it's going to be YOUR face it blows up in.
If necessary to repeat that EXACT message to the Head Coach (Administrator, Superintendent of Schools and anyone else), as discreetly as possible.