If you cannot disable it from the CP (CP -> Portal & Islands -> Column Layout) you can edit your includes/config.inc.php:
Code
  'LEFT_COLUMN_BOXES' => 
  array (
  ),
  'RIGHT_COLUMN_BOXES' => 
  array (
    1 => 'portal_box_8',
    2 => 'public_calendar',
    3 => 'portal_box_4',
    6 => 'portal_box_7',
    9 => 'forum_stats',
    12 => 'top_posters_30',
    19 => 'portal_box_39',
  ),

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