Remove Category option from Layered Navigation options list

Try this:
1) open app\code\core\Mage\Catalog\Block\Layer\View.php
2) find

$categryBlock = $this->getLayout()->createBlock('catalog/layer_filter_category')
            ->setLayer($this->getLayer())
            ->init();

$this->setChild('layer_state', $stateBlock);
$this->setChild('category_filter', $categryBlock);

3) change with

$this->setChild('layer_state', $stateBlock);