This question was asked on Quora. "Do I need to put quotes around single negative keywords in AdWords?"
Here's my answer, saved for posterity.
If you put quotes around the negative keyword it makes it a phrase match negative keyword. Phrase match negative keywords stop ads from showing when the search contains the words in the same order.
If your negative keyword is a single word then it wouldn’t matter whether you have quotes around it or not because it would stop all searches containing that word.