I don’t know what your local ordinances are, but usually a private business has to post a no firearms sign to make it a gun free zone.
People who are in charge of private property can authoritatively ask some one to leave. It is the essence of private property. If you cannot ask people to leave, it is not private anymore.
Wisconsin just passed a significant weapons reform bill, SB93. I expect that Governor Walker will sign it Friday the 8th of July. It clarifies much of the law and is a large step toward Constitutional carry.