So, i'm looking for some ideas on dealing with a character (not a player) that has made up his mind to do something totally and utterly irrational, even though he is not, in fact, an irrational person in any other way, and every time any evidence, no matter how strong or who provides it--other PC's, NPC's, the people he's trying to effect--let him know that it is an irrational idea, he only responds with, Meh, they're lying to me, this will work.
I've spoken with him OOC about this and his basic premise is, well, he's decided this needs to happen aaaaand he's going to make it happen. This boils down to, he's going to do it no matter what cuz he wants to do it. This line of action would directly:
1) remove him from the party
2) cause serious hardship for the party/world (i'm fine with that)
3) cause a significant amount of in game time to be spent dealing with JUST him, as no one else in the party will go along with his genocidal plans.
I'm fine with him rerolling and screwing things up in the world, but, the hijacking of sessions (and i can't even do a side session with just him bc the others will likely try and stop him, and he's not really trying to hide his actions from them) just irks me. I'm strongly
against the whole, GM Fiat, No, you cannot do that, stop exploring this line of action, because it is antithetical to my ideas of GM'ing. That said, i don't want to spend multiple sessions dealing with him putting his genocidal plans into place.
Anyone have thoughts on how to deal with this without leveling a mandate of, NO?