The file name of the NOA dictionary is
B003ODIZL6_EBOK.azw. But simply deleting it won't do you any good, because the Kindle app will simply re-download it the next time you look up a word.
If you want to use an alternative dictionary, you'll need to change its ASIN to
B003ODIZL6 so that it matches the NOA dictionary.
For more information see this
thread.