when you only delete the outer text you delete the HTML content itself, but if you perform another find on the same elements it will appear in the result. the reason is that the simple HTML DOM object still has it's internal structure of the element, only without its actual content. what you need to do in order to really delete the element is simply reload the HTML as string to the same variable. this way the object will be recreated without the deleted content, and the simple HTML DOM object will be built without it.
here is an example function:
public function removeNode($selector)
{
foreach ($this->find($selector) as $node)
{
$node->outertext = '';
}
$this->load($this->save());
}
put this function inside the simple_html_dom class and you're good.