You'd need to disable the JFBCSystem plugin to disable most functionality of JFBConnect. The simplest way would be to edit the following file:
/plugins/system/jfbcsystem/jfbcsystem.php
Around line 34, you'll see:
function __construct(& $subject, $config)
{
parent::__construct($subject, $config);
Update it like the following:
function __construct(& $subject, $config)
{
return;
parent::__construct($subject, $config);
The "return;" statement will prevent the plugin from running. Once you do that, you should be able to access the admin area again.
As for your issue, that shouldn't happen with JFBConnect v9. Can you let us know what version of JFBConnect you have installed and any other details that may help us localize the issue? We have not heard of similar problems with the latest release but will do whatever we can to help get you going!
Thanks,
Alex