I think there's always going to be someone you can't get along with. That's part of being human. Sometimes, people just click, both in the best way and occasionally in the worst. Magos and lantz, you and lantz, you're just rubbing each other the wrong way, and in this case I don't think any amount of politeness or attempts at discussion are going to change that.
Leaving aside who's in the wrong here, in an attempt to be slightly impartial, I think we can all agree that some people are more stubborn than others. lantz is possibly one of the most stubborn, thick headed people I know besides my mother, which is neither good nor bad on its own. Hell, stubborn people run this world. Sticking to your guns no matter what is considered a good thing in many places. In this situation, though, I'd say that he's simply not going to listen to you, ever. lantz was in BL before I arrived, giving him more than 3 years of experience in dealing with tough and competent admins, Elf among them. He, like Mike, is not going to change because of someone else telling him that he's wrong. It's just not happening, and all the well thought out arguments in the world won't help you. You're not the first who's getting worked up because of his actions, and you won't be the last.
So I say calm down. You've done some good work here. This forum is growing. This RP has escaped the BL RP curse and is actually continuing for more than a month with still frequent posting. You've attracted good posters like YOLF, Aiden, and, dare I say it, myself to come over. You've made some damn fine porn with a published writer, and the World of Darkness group is having fun roleplaying with you every Saturday or Sunday. That's something.
As for dealing with lantz and how he makes you feel, I'd say to leave your interactions mostly to the IC thread. Since arguing here is only going to make him more stubborn and irritate you, don't do it unless you have to. He can argue against you, but he has to react to your in character actions, and we'll call him out on it if he does any kind of bullshit. This way, you can gain some emotional distance from the situation and only deal with him when you have to, rather than getting sucked into the debate and stressing out every time he does something horrible, which he'll keep on doing no matter what you say until he's banned or kicked out of the RP.
As for the warning, only you and Elf can decide if it was justified. I highly doubt lantz will ever get another one for quite some time, since he's good at toeing the line and being just reasonable enough to not get himself punished for it. For now, just relax and have some hot chocolate. It's freezing out there.