That title probably doesn't make a lot of sense, the idea sounded better in my head. Like saying no to snobs... Anyway let's move on from my crappy titling skills.
Snobby people are the type of people that make me roll my eyes every time they open their mouths, and they can be extremely annoying. They're either commenting on the fact that you're clothes aren't a designer brand, or they're commenting on the type of music or books you buy, or they're just looking down on you in general.
I hate to give this advice because it sounds so stupid and I know it's not this easy, but IGNORE THEM. Snobby people only comment on what other people do in their lives to validate their own existence and their own purchases. She's probably commenting on your shoes because she's insecure about her own clothes and wants to make sure everyone knows that she looks better (which she ultimately doesn't because designer clothes does not always equate to coolness).
Just nod, maybe mutter a 'uh-huh' and move along. Don't try and fight someone over this, because at the end of the day it reflects more on that person than it does on you.
If you give into their words, you'll find yourself trying to change who you are and that is never okay. Just pretend like they're speaking in another language, and let it all bounce off you.
So rule thirty five of surviving Teenagehood: if anyone approaches you wearing a monocle and a smug grin, get ready to tune them out.