MC_NPC_GET_PLAYER_TARGET

[jerbmega]

passes npc and player, return false to invalidate a player as a valid target

[ConnorForan]

Probably relevant note, there is the MC_NPC_PICK_TARGET callback that runs after the whole target selection is completed, provides the npc and the target that the game chose, and allows returning a different entity to change the target.

It can't do the "invalidate a player as a valid target" thing (apart from just returning a different target yourself), which would probably require patching into the spot where the game decides whether or not a given player is a valid target, so that it could go on to choose a different target? Though there's also stuff like familiars or other enemies being selected as the target to take into account.