Deploying a robots.txt file for Hyrax is synonymous with deploying it for Tomcat. This means that your robots.txt file must be accessible here:
http://you.host:port/robots.txt
For example:
http://www.opendap.org/robots.txt
Note: Placing robots.txt lower in the URL path does not seem to work
In order to get Tomcat to serve the file from that location you must place it in $CATALINA_HOME/webapps/ROOT.
If you find that you system is still burdened with robot traffic then you might want to try the BotBlocker handler for the OLFS.