There isn't a way to disable the Open Graph or Twitter tags on the page. That's something that's simply enabled right now. We'll have to test with Jumi more to understand what's going on there. Our Open Graph tags are generated at multiple steps during the page rendering based on various factors (like if you have an Open Graph plugin enabled for the specific component you're using).
What I'd recommend is:
* If the component generating that page is Joomla Content, or another component with an Open Graph plugin, disable that plugin.
** This will make JFBConnect use the meta description tag for the page instead of trying to scan the article contents to generate a 'better' description.
* Ensure the JFBCSystem plugin is ordered after the Jumi plugin. Depending on when Jumi does it's replacements, that may help, but likely not from what you describe.
Disable the Open Graph tags with a minor code change. To do so, edit the /plugins/system/jfbcsystem/jfbcsystem.php file. Around line 275, you'll see:
$this->replaceGraphTags();
Comment that line out and not tags should be inserted on the page.
I hope that helps,
Alex