We don't have a 'block reconnections' setting, but you can basically disable it depending on your configuration.
Mainly, in the SCLogin module, make sure the "Show Connect Account(s) Buttons" setting is disabled. That won't show the option for the user to connect accounts when they are logged in.
Then, in the JFBConnect configuration area, if you're using the "Normal" account registration mode, set the "Registration Display Mode" to "Register Only". That will make it so users can't link to an existing Joomla account. If you're using the "Automatic" registration mode, that setting won't matter (or be visible).
If you're having other areas where users are being offered to connect their accounts that you don't want, let me know. There's a few ways users can re-connect accounts, but the above should effectively hide those options.
Thanks,
Alex