Where part of an aboveground swimming pool structure is used as an enclosure or where the enclosure is mounted on top of the aboveground pool structure, and the means of access is a ladder or steps, then the ladder or steps shall be capable of being secured, locked or removed to prevent access, or the ladder or steps shall be surrounded by a enclosure which meets the requirements of this section. When the ladder or steps are secured, locked, or removed, any opening created shall not allow the passage of a four-inch diameter sphere.