Android Popping off the Activity Stack

Use the android:noHistory property in the manifest:

http://developer.android.com/guide/topics/manifest/activity-element.html

If A is already running, you can use the FLAG_ACTIVITY_CLEAR_TOP flag when starting an intent to go back to A.

See also, similar questions:

How to clear current activities in the stack?

how to kill sub activities and bring activity to top of stack