While it's true that some police officers do carry a gun, not all do so. Your original call was "Don't sell to police", not "Don't sell to those carrying a gun.

While it's true that some police officers do carry a gun, not all do so. Your original call was "Don't sell to police", not "Don't sell to those carrying a gun." You are using that as a proxy workaround. However, it likely won't work as I believe the laws regarding "no firearms allowed" signs exempt law enforcement from following those signs. (Consider, for example, the difficulty of entering a business where the business owner is sniping passer-bys and has also posted "no firearms allowed" signs on the entrances.) FWIW, the Unruh Civil Rights Act is broader in scope than Title III of the Civil Rights Act, which I believe is what you are referring to. For example, it also includes discrimination against someone "who wore long hair and dressed in an unconventional manner". My point remains: it may be illegal for a business to discriminate against someone based on their occupation. It's best to check with the laws before carrying out your proposed social rejection scheme, if only to have an idea of the possible consequences.