Been there. While the sign has generated some local controversy and mirth, I've gotta say ... if you are annoyed to that extent by the Mission District's considerable "hipster" population (most of whom don't actually live there, but congregate en masse daily) then why did you set up a high-quality coffee shop in the most hipster zone of all San Francisco? That would be sort of like setting up a dim sum restaurant in Chinatown and insisting "only English language permitted!"
Anyway, it's funny. While I find the current crop of Mission hipsters to be annoying, I defend them anyway, because at least they bring money to the 'hood and aren't gangbangers. On the other hand, they are seriously diluting what used to be a much more "ethnic" neighborhood, so it's debatable whether they're a boon or an annoyance. The smugness, obliviousness, entitlement and enormous crowds are a drag to cranky locals like me, but on the other hand, they're ultimately pretty much harmless. I'm perfectly happy to consign them to the coffee shops and the wrong ice cream line (Bi-Rite) but get the heck out of Duboce Park, you kids!
Argh!