Don't try to be smart, don't try to be sarcastic or witty... just don't respond. Keep training. When they see they can't get a rise out of you, they'll go away.
The moment you engage them, they've disrupted your training and then they've won.
So just train.
It's hard to ignore people punching you
One reason one of my friends doesn't do parkour is this:
Same thing as you, except when they couldn't do it they were scared of looking pathetic and pretty much jumped him.
(Which is ironic, they were trying to not look pathetic, and made themselves pathetic.)
But yeah, he's paranoid now.
What I would do is play it off.
Act like you would if it was friends kidding around with you, and wait for the "It's so easy part" and say something like "Yeah, show me how you do it, I would love to see your form and learn from it."
Don't sound like a smart ass though, cuz that's more of an insult.
This is what I would do, not what you should do, as really, most likely it wont get you any farther.