No clue why FN is acceptable, but to make it unacceptable, you'd need to
create an "exclude dictionary". The exclude dictionary contains words that
Word thinks are okay, but which you want flagged as misspellings anyway. In
the Help search, look for "exclude dictionary" for the instructions on how
to create one (it can vary by Word version). Here are the instructions from
the Help file for Word 2003:
To specify a preferred spelling for a word, you add unwanted variations of
the word's spelling to the exclude dictionary:
1.. Click New Blank Document on the Standard toolbar.
2.. Type the words you want to include in the exclude dictionary. Press
ENTER after each word.
3.. On the File menu, click Save As.
4.. In the Save in box, browse to the following folder:
C:\Documents and Settings\user name\Application Data\Microsoft\Proof
5.. In the Save as type box, click Plain Text.
6.. In the File name box, type a name for the exclude dictionary.
Make sure to give the exclude dictionary the same name as the main
language dictionary it's associated with, except use the file name extension
..exc. For example, the English (United States) dictionary is called
Mssp3en.lex, so name the associated exclude dictionary Mssp3en.exc.
Also make sure that the file extension is correct— for example, that .txt
isn't appended to the file name extension.
7.. Click Save.
8.. In the File Conversion dialog box that appears, select the options you
want, and then click OK.
9.. On the File menu, click Close.
10.. To activate the exclude dictionary, you need to restart Microsoft
Word.
