The Login (and all the other modules) have an option for "Show Powered By Link". If you set this to 'No', the link will vanish.
Back-links obviously help us generate more traffic to our site, which helps get more interest in JFBConnect, which helps add more subscribers, which helps pay the bills and develop more features. However, we hate 'mandatory' links, and will always provide an easy opt-out ability with our paid versions. If that option doesn't work as expected, definitely let us know, and we'll fix it!
Additionally, if you are leaving the link-backs, you can always take advantage of our affiliate program, where you can earn a commission on any users you send our way that subscribe. We have an easy option to add your affiliate ID to the modules too!