This is worth noting for posterity.
When I train my cats, I do it simply. For NO: A stern voice, a light tap if needed, a squirt with the water bottle if necessary. Always an immediate change of demeanor when they do what I'm asking, a pet and thanks, and a slow blink of the eyes to tell them in their language that they did good.
First example:
Mambo loves to attack my fingers when we're playing. I don't want him to. So I tell him NO and stop petting when he opens his mouth to grab me, and praise him when he shuts it again. Up until very recently, this had no effect; he'd still grab those fingers.
The past few days, he's made like he's going to grab my fingers, then looks at me to see what I'll do, then licks them instead.
And yesterday, a whole play session with NO biting, ONLY licking. Success!
Second example:
Mambo loves to attack Dante from out of nowhere. They can't be close to each other without Mambo going for Dante's neck. Playing would be fine...if Dante were able to get used to just having the little guy nearby without being attacked. So whenever I can, I sit with the both of them nearby and pry Mambo's teeth from Dante, telling him NO, and talking nice to him when he retreats. Over and over and over again, usually for a minute straight.
Today, for the first time, from the other side of the couch, he stopped at my "NO" before grabbing Dante's neck, and curled up purring instead, begging to be petted.
(Yes, I know rough play is all part of being a kitten, but Dante will never play with him if he keeps doing this at every opportunity. Dante's attitude at this point is not "Okay, I'll play with you," it's "Get the fuck away from me before I get REALLY pissed.")