You need to use the method cancel(...) from AlarmManager, using the same PendingIntent you used to set the alarm. Example:
this.getAlarmManager().cancel(mAlarmPendingIntent);
(this refers to the Activity or the Service from which you are cancelling the alarm).
Create the PendingIntent as:
mAlarmPendingIntent = PendingIntent.getActivity(this, requestCode, intent, flags);