/nopk player
From Nightmist wiki
General
To use this command, you simply put "/nopk (player)" where the "player" is the one in which you wish to add or remove. Using this command once will add them; a second time will remove them.
Players on this list will not be able to be attacked unless they hit you first. Just because you add someone to your list doesn't mean they won't be able to hit you, so make sure they add you to their Nopk list as well.
The status of who is or isn't on your Nopk list can be checked by the /nopk command. This command will also show you who has you on their list.