If you wish to disable scrolling you must deselect the "Vertical Scrolling" and "Horizontal Scrolling" check box in the Mouse tab of the Keyboard & Mouse Preference pane of System Preferences. Selecting "off" in the Scroll Ball button pop-up menu of this preference pane only disables the Scroll Ball button feature.
Mighty Mouse: How to disable scrolling
The Scroll Ball has two functions; to provide scrolling in all directions, horizontally, vertically and diagonally and to act as an additional button.
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