Generally speaking, you can handle this in-character or out-of-character. 

It seems like you've already tried reasonable enough in-character methods, so what you are left with is escalation in that vein-- NPC takes more and more offense, NPC allies materialize to deliver messages, NPC eventually withholds critical assistance to the PC or party due to poor behavior, etc. 

My read, here, is that this won't work, because your player is just bound and determined and may just be willing to soak up as much in-game punishment as it takes to pursue this, uh, quest.  I am generally hesitant to turn this into a collective punishment situation, because it lends (or at least, can lend) an unhealthy GM vs PCs vibe to a game if handled wrong, but this might be a situation where it is warranted. 

Another angle is for the NPC to enlist the other PCs in this drama directly-- surely they have opinions, having seen someone get killed by a Balor as a result of these shenanigans.  

The other option is out-of-character, and breaks down along the same lines of do it yourself or enlist the other _players_ rather than other characters.  And in either approach, the player needs to be made to understand that he is not the sole determiner of what is and is not OK in terms of behaviors like that.  In a game-realism angle, the NPCs and other PCs vote; in a game-as-social activity, the GM and other players get votes.  In either case, this is just as disruptive (say) as a thief who keeps stealing from important NPCs, a Lawful-Stupid Paladin or relentlessly aggressive barbarian who keeps insulting important NPCs, etc. 

["My Guy" syndrome][1] (i.e., "But that's what My Guy would do!") is not a defense, it is not okay, so knock it off would be the basic message.  

Finally, while it may or may not be the fundamental issue here, I will make this point as well:  If I saw this behavior and dynamic going on between two PCs (one aggressive and self-righteous, one not interested and clearly wanting it to shut down) I would with 110% certainty consult the second player and if needed, step in and intervene.  It is harder to be objective, but no less valid for a GM to intervene on behalf of him or herself if the situation is causing personal discomfort.  


  [1]: https://rpg.stackexchange.com/questions/37103/what-is-my-guy-syndrome-and-how-do-i-handle-it