I recently checked my stats page and noticed an anomaly-googlebot is consuming about 30GB of bandwidth on my site per month.

if I wanted to block googlebot from www.mysite.com/directoryA/fileZ.php, how exactly would I do so? What would robots.txt look like, and where would I put robots.txt, in the directoryA directory?

If it's just googlebot I'd setup a google site map.. That way you can tell it how often you expect the content on each page or area to be updated. Then you don't waste any bandwidth.

http://www.robotstxt.org/

http://www.sitemaps.org/protocol.php

wow thats a lot of bandwidth, make robot text file and put it in dir as files, i still cant belive it ate that much though amazing

For me it consumes even more :). But google bot is always welcomed in my site :)