Just copy the following code into your child theme’s functions.php:

function remove_widgetized_area($content) {
	unset($content['Secondary Aside']);
	return $content;
}
add_filter('thematic_widgetized_areas', 'remove_widgetized_area');

Here’s a list of the default widget areas:

• 'Primary Aside'
• 'Secondary Aside'
• '1st Subsidiary Aside'
• '2nd Subsidiary Aside'
• '3rd Subsidiary Aside'
• 'Index Top'
• 'Index Insert'
• 'Index Bottom'
• 'Single Top'
• 'Single Insert'
• 'Single Bottom'
• 'Page Top'
• 'Page Bottom'

Please use the ThemeShaper Forums for any further questions.