I would read the quick excerpt version of the "Luby's Massacre" story. Explain to them that someone who wants to do someone harm will do so regardless of whether or not they post signs. And this is a case in point. I would even go so far as to say that establishment was chosen by the murderer because it was a "gun free" zone.
"On October 16, 1991, 35-year-old George Pierre Hennard, an unemployed merchant seaman who was described to others as angry and withdrawn, with a dislike of women, drove his 1987 Ford Ranger pickup truck through the front window of a Luby's cafeteria at 1705 East Central Texas Expressway in Killeen, yelled "This is what Bell County has done to me!", then opened fire on its patrons and staff. He stalked, shot, and killed 23 people while wounding another 20 before committing suicide by shooting himself. During the shooting, Hennard approached Suzanna Hupp and her parents. Suzanna had a handgun in her vehicle outside, which she was unable to bring in because weapons were not allowed in that restaurant. Her father charged at him in an attempt to subdue him but was gunned down; a short time later, her mother was shot and killed.
Responding to the massacre, the Texas Legislature passed a shall-issue gun law, which requires that all qualifying applicants be issued a Concealed Handgun License, removing the personal discretion of the issuing authority to deny such licenses. The law had been campaigned for by Suzanna Hupp, who was present at the massacre where both of her parents were shot and killed. She later expressed regret for obeying the law by leaving her firearm in her car rather than keeping it on her person."