how to send out an activity from the Continuum of activities

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Java

Mobile

Android

fact :
as you know we can move between activities if we use the Intent . they are like a chain .
problem :
i created an Activity in my application . this Activity is used for login .
information are Sent to server , if they are Correct the Second Activity appears.
but every device has a back button . if user clicks on back button the previous Activity appears!
Question :
how can i Destroy َthe previous Activity ?
users dont have to see previous Activity