To report mail as spam:

In MobileMe Mail on the web, select a message marked as junk (with a brown header), and click the “Report as Spam” button in the upper-right corner of the header.

If it’s not already in the Junk folder, the message is moved to the Junk folder.

If you see a Not Junk button but don’t see a “Report as Spam” button, you’ve already reported this email as spam. (If you don’t see either button, this email is not yet marked as junk; you need to mark it as junk before you can report it as spam.)

You may continue to see junk messages from a specific sender even after reporting one as spam. However, continuing to report new junk email as spam contributes to reducing junk mail messages.