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that page is joomla default for login page. It cannot disable it (becuase jomsocial running under joomla). but you can changing default user registration to jomsocial registration.
goto jomsocial redirect plugin at joomla backen > plugins manager and Override com_user registration = Yes
to disable system message:
.alert-message {
display: none;
}
put that CSS code at /teamplates/socialize/css/custom.css