Doubts related with a app with login page

I have this sample:

Office365RESTAPISample

That has a login page, contact view and edit contact view, the flow is:

LoginView > ContactView > EditContact

And I have some doubts:

Should I have to remove the login page from stack of navigation after I navigate to ContactsView, or should I keep it?
In my idea, I think after I do login and navigate to contact view and then do back using hardware button, the app should close and not navigate to login page,... or I am wrong?

If i need to remove the page from navigation stack, how I do this in Xamarin Forms?