SpamCop is the premier service for reporting spam. SpamCop determines the origin of unwanted email and reports it to the relevant Internet service providers. By reporting spam, you have a positive impact on the problem. Reporting unsolicited email also helps feed spam filtering systems, including, but not limited to, SpamCop's own service.
This service is embedded in the web based squirrelmail MUA, check you squirrelmail options for this.