How to kill an Android activity when leaving it so that it cannot be accessed from the back button?

You just need to call finish()

Intent intent = new Intent(this, NextActivity.class);
startActivity(intent);
finish();

Setting android:noHistory="true" on the activity in your manifest will remove an activity from the stack whenever it is navigated away from. see here

you can use:

intent.setFlags(Intent.FLAG_ACTIVITY_NO_HISTORY);