To prevent the GSA from crawling a certain directory of your web server, it is best to use a robots.txt file. See The Web Robots Pages (http://www.robotstxt.org/wc/exclusion-admin.html) for a discussion of robots.txt.
Sometimes you can not change the robots.txt file because you do not administer the web server. If this is the case, you should block the GSA with <META> or googleon/googleoff tags.
What is indexing?
How do I prevent individual web pages from being indexed?
How do I exclude unwanted text from being indexed?
The Web Robots Pages