In Dutch we have this existing word 'nar' that isn't very frequent. However, as a typo for 'naar' (= to/in the direction of/towards) it is quite frequent.
After reading about adding a star to the entries of your main Hunspell dictionaries, I decided to test the effect: indeed, a word with a leading star (*) is flagged as a typo:
Note that you have to edit the main Hunspell dictionaries, not the custom dictionaries.
And make sure that you remove any existing entry for the word to flag.