Remove option to login with Facebook from site dashboard?

Just worrying that having the facebook login option on my site's admin dashboard may be a security threat? How do I remove it from the dashboard page please?

Also, how do I ensure that all new users will be unable to achieve admin status? The Facebook login thing makes me a bit nervous, as I'm not sure where the identity of these members are stored once registered on my site, and I don't know which user role they are assigned to?

Thanks