Open the qa-page-not-found.php file and find this text:
$qa_content=qa_content_prepare();
$qa_content['error']=qa_lang_html('main/page_not_found');
$qa_content['suggest_next']=qa_html_suggest_qs_tags(qa_using_tags());
First line leave it as it is. You can remove any of the other 2 lines as you please or even add new HTML code in this way:
$qa_content=qa_content_prepare();
$qa_content['custom_not_found']='I am a nice piece of <strong>HTML</strong> code :)';