SPRINGFIELD — The Illinois Supreme Court is being asked to decide the extent to which property owners can block anyone else, including neighboring property owners, from accessing a river that runs through their property.It’s a question of law that Illinois courts have not fully addressed before and one over which different states have vastly different policies.The case involves two property owners who own land along the Mazon River, in Grundy County, a portion of which has been declared a National Historic Landmark because …