Question : Mail server is used by someone to send spam. How to stop that?

Web provide a sharing a hosting on our server.   Now we got complain that there are a lot of spams are sent from this server IP.  
Have no idea how to stop that.
We use Imail.   In mail SMTP is set to "No Mail Relay".   How do that send out those spams.

Answer : Mail server is used by someone to send spam. How to stop that?

Solution: After checking the server it was discovered that an anonamizer style proxy service had been installed on the server which was allowing malicious users to connect through this server and onwards to other servers on the net. We removed this proxy and tightened overall security on the server.