When you disable Lightbox for the forums, it doesn't disable it for the tutorials application. To fix this, edit the skin_tutorials_display -> viewArticle skin template, find this code:
{parse template="include_lightbox" group="global" params=""}<if test="disablelightbox:|:!$this->settings['disable_lightbox']">{parse template="include_lightbox" group="global" params=""}</if>