While I'd normally have just have thrown this encounter in my ever-expanding "Fuck vegans, amirite?" pile, there was something about this particular guy that made me realize for the first time (I am not a very smart man) that this kind of obnoxious behavior is less about the school of dieting a person buys into and a lot more about the person himself. The reason people like me think vegans are punchable dickholes isn't veganism itself; it's the relatively small percentage of preachy assholes within the movement. I call this specific brand of jerkwad the Obnoxious Fanatic, and he/she pops up in each and every phenomenon, trend, and philosophy as reliably as Rule 34.
You don't hate hipsters because of the hobo-looking folks minding their own business at the corner of a cafe in Williamsburg -- you hate them because of those couple of Pabst-drunk fuckers you once met at a party, peacocking their sailor tattoos and actively condescending to everyone. You don't treat yoga suspiciously because of the discipline itself; it's because of that one co-worker who keeps sending you links to her blog, "Yogifreek," whose every post ends with the word "Namaste" and features copious amounts of yoga pants buttcrack. The Obnoxious Fanatics worm their way into everything and bang their drum in every direction. Because of them, our general attitude towards most trends can be summed up by a joke I once heard: "A CrossFit trainer, a gluten-free dude, and an atheist walked into a bar. We know this because they told everyone."
And that's the point, really. Unless you're into, I don't know, animal mutilation or Gamergate or something, chances are the vast majority of humanity is completely cool with your little hobby/philosophy/passion. Where you rank on the official Poisuo Asshole Scale highly depends on how little you can shut up about it.
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It can also be used to measure actual assholes, usually right before I force the metaphorical ones to climb in.
How To Deal With Them:
Convince them to avoid dealing with you.
Your average Obnoxious Fanatic is, for most intents and purposes, a fundamentalist. They demand a strict adherence to certain doctrines, ridiculous as they may seem. As everyone who's ever attempted to argue against one can attest, it's ... slow going, to say the least.
So don't argue with them. In fact, don't engage at all if you can avoid it. Their bullshit opinions can't hurt you. They shouldn't even be able to annoy you, much as they try. Just assume your finest thousand-yard stare until they get the hint and wander away to spew bullshit at someone else.
And if push really comes to shove, then remember: Other than their fanaticism about a certain thing, they're more or less like you, which means they hate obnoxious fanatics too. Look deep into your soul and find the most boring, pitiful thing you're passionate about, and unleash it upon your opponent. I guaran-damn-tee that even the most obnoxious fedora atheist will flee within minutes when you start preaching the joys of collecting Pokemon-themed postcards.