Are you sure the head is designed to hold that much weight? Many ball heads won't properly support a larger format camera.
And are you talking about the part where the tripod head meets the body? If that's where it's spinning, and the adjustment screw is fully taught, then the entire head my be spinning over the body. Usually, there are some set screws bellow the head that keep it from spinning uncontrollably. If not, make sure the head is screwed all of the way down onto the body, and maybe try a bit of temporary thread locker (not the permanent stuff in case you ever want to remove them from each other in the future).