If you go to Manage Account (should be under Profile), there should be an option to set a 'Sign-In- password' which will let you set a 4-Code password that uses the controller.
I.E. [RT], (Y), [LB], (Y)
So that when you choose to sign in, you have to enter the passcode for that Gamertag.
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If you can't find Profile, hit your Guide button (big button with Logo on it), go all the way right, and there should be 'Profile'.