How to Show Categories in Left Sidebar in Accordion View

I had same problem, after some coding I made this in template/catalog/navigation/left.phtml:

first line:

<?php $ct = Mage::registry('current_category')->getID(); ?>

in the end of the file(before endif;)

<?php elseif( $ct == 92 || $ct == 85 || $ct == 224): ?>
<?php
$currentcategory = Mage::registry('current_category');
$subcategorys = explode(',' , $currentcategory->getChildren());


?>

<div class="block block-layered-nav">
    <div class="block-title">
        <strong><span><?php echo $this->__('Browse By') ?></span></strong>
    </div>
    <div class="block-content">
        <dl id="narrow-by-list2">
            <dt><?php echo $this->__('Category') ?></dt>
            <dd>
                <ol>
                <?php foreach($subcategorys as $cat): ?>
                     <?php $cate =  Mage::getModel('catalog/category')->load($cat); ?>
                    <li>

                        <a href="<?php echo $cate->getUrl()?>" <?php if ($this->isCategoryActive($cate)): ?> class="current"<?php endif; ?>><?php echo $this->escapeHtml($cate->getName()) ?>(<?php echo $cate->getProductCount() ?>) </a>                  
                    </li>
                <?php endforeach; ?>
                </ol>
            </dd>
        </dl>
        <script type="text/javascript">decorateDataList('narrow-by-list2')</script>
    </div>
</div>

<?php endif; ?>

Via layout XML (although this will remove the whole block including the container):

<remove name="left" />

Programmatically: In your config.xml, listen to the event core_block_abstract_to_html_before (this will keep the container, but will remove all child blocks):

<core_block_abstract_to_html_before>
    <observers>
        <your_extension>
            <class>your_extension/observer</class>
            <method>coreBlockAbstractToHtmlBefore</method>
        </your_extension>
    </observers>
<core_block_abstract_to_html_before>

And then in your Observer.php, unset all children if the current block is the block of the left sidebar:

public function coreBlockAbstractToHtmlBefore(Varien_Event_Observer $observer) {
    /* @var Mage_Core_Block_Abstract $block */
    $block = $observer->getEvent()->getBlock();
    if ($block->getNameInLayout() == 'left') {
        $block->unsetChildren();
    }
}