How to prevent going back to the previous activity?

Question

When the BACK button is pressed on the phone, I want to prevent a specific activity from returning to its previous one.

Specifically, I have login and sign up screen

Answer 1

Put

finish();

immediately after ActivityStart to stop the activity preventing any way of going back to it. Then add

onCreate(){
    getActionBar().setDisplayHomeAsUpEnabled(false);
    ...
}

to the activity you are starting.