You're talking about punishing wrong-doers. Of course! But the real point is not punishing the rest.<p>At CD Baby, we had thieves with stolen credit cards every single day.<p>We'd usually catch them through a quick real-human scanning process we'd do before packing any orders.  (Is it out of character? A $1000 order going to Nigeria? Etc.)<p>About once a week, someone got away with it.  We'd constantly improve our methods, but always behind-the-scenes, never making shopping hell for the honest.  Just made it our responsibility to deal with the thieves without bothering the rest.<p>In your coffee shop example: if someone is stealing or screaming then yes of course you deal with that person when it happens.<p>But no need to accuse everyone and make coffee hell for everyone else.<p>Besides, no plastic sign will stop the immoral or insane.