To remove JomSocial's logout, you'd have to modify the JomSocial templates to remove it. I honestly don't know all the places that you'd have to remove this from off-hand, but could help you if you can't find them. Hopefully, there's just one file that you can edit that would remove it on all pages.
The other option is to disable the "Automatically log Facebook Users In" in the JFBConnect administration area. That way, if a user logs out using the JomSocial logout button, they won't be logged out of Facebook, but they won't be automatically logged back in.. efectively logging them out of your site.
Hope this all helps! Finally, if you use JFBConnect please consider posting a rating and a review at the Joomla! Extensions Directory:
extensions.joomla.org/extensions/social-...ook-integration/7215
Thanks, and good luck!