Nessus a handy tool for attackers!!!

The most significant incidents on my IDS/IPS sensor in the past two weeks were the logs which showed someone is trying to find out about any vulnerability on our corporate website and portal and more interestingly, this guy didn't care about any customer websites and was seriously working on our corporate webserver which has many meanings to me.
He/She was using NESSUS to scan our server and find out about any vulnerability on our operarting system, web server and Email System.
When fronting yourself with these cases the worst part is that you can not take any legal action against these guys! You may block their source IP address temporarily but at the end you can only carefully watch their activity and pray!!!