Just dont give them your business. I couldn't care less if the owner "knew" that my money was being spent at their location. If the owner went through the trouble of creating and posting a No Firearms sign, I'm sure they aren't going to be phased by a letter expressing ones issue with not being able to carry inside the establishment.
" Oh, my God. They found me. I don't know how, but they found me. Run for it, Marty! " - Doc Emmett Brown