I just add them as I find them. The are other ways to do this also.
This is a good site for various .htaccess code http://www.askapache.com/
Code:
RewriteEngine on
Options +FollowSymlinks
RewriteBase /
########################################
##################################
RewriteCond %{HTTP_USER_AGENT} "Gigabot"
RewriteRule .* - [F,L]