From Xojo Documentation
The user has clicked on the passed column header to sort that column or the Sort method was called. Column is zero-based.
Note that sorting is based on string comparisons. If you want to sort numbers, dates or CheckBoxes then you have to use a custom sort in the ListBox.CompareRows event.
If you're trying to prevent the user from sorting the column in the first place, use ListBox.ColumnSortDirection.