Report spam to our abuse team
The best way to report abuse is to view the email's full header information, and find the unique campaign ID that we embed into every single email that's sent from the Mailchimp system. This campaign ID (CID) helps us track down the exact user and offending campaign, then take appropriate actions. This is an illustration of how to find our abuse tracker, and here's the form to submit the CID.
Don't see the unique campaign id in the full headers? Mailchimp was probably just "spoofed" by a virus or spammer out there pretending to be us.