As I know, there is no proper way to disable this icon. It is possible to remove "Go to Admin panel" text and icon using following article: http://kb.odin.com/en/4634.
As for the "Already a user..." text it is only possible to just remove this text from the locale file and it will disappear from the screen but Login icon will be still there. But you can try it:
/usr/local/sitebuilder/resources/locale/en_US/Wizard/Common.lng
....
login = "Already a user? Log in here..."
disabling going to admin panel,there is no
disable_go_to_admin_panel selection on Sitebuilder 4.5 for Linux,
remote_admin = on
is instead of this selection?