Well, in case it's not above like 800 words you should've no trouble counting them yourself and excluding certain words yourself
If however it is some kind of "book" you've written there, I don't have the slightest idea how to exclude words.
You could, however make a list (on paper) for every word that shouldn't be counted, then save another copy of the essay, search for every appearance of every word on your list, erase them all, and then look at the word count.