I don't think there's much more you can do... You state NK's parents are also a known problem, which makes me think his parents aren't going to do anything about it even if you talked with them about it.
As far as playing on another neighbor's playset, where the neighbors home? If they weren't, I'd let them know that NK and your kids were there playing, and make sure that's okay... But, I wouldn't let my DC play there if no one is home, as I always wonder what would happen if DC fell, broke an arm, if the playset fell and injured DD, etc., and if home insurance would cover injuries if the owners weren't home, and the kids weren't there with the owner's approval.
That being said, I would try to (severely) limit my DC's time with NK... and since NK has shown to be disrespectful to me, he wouldn't be allowed to play at our house/our yard. It would make things suck if he's the only NK, I still would stand strong. I would also discuss his behavior with my DC, saying something like, "What do you think of what NK was saying today? Do you think that's a nice way to talk? Do you think you would like friends who talk like that? Was that respectful? Would NK like you saying that about his parents?" Just kind of a check-in so that they know it's not acceptable to talk like that, that your family has higher expectations for behavior, and that NK's comments weren't how a good friend would talk.
--Mimi
Mom to Lala (2004), Bonus Mom to Big Sis 1 (1991) and Big Sis 2 (1992)
Grammy to Big Kindy Kid (2011), Big Pre-K Kid (2012),
Grandbaby Appendage (2014), and New Baby Grandboy (summer 2017)