This is an old revision of the document!
This is an Automator Service that sends the full source of a selected Mail message to the address of your choice (like spam@myISP.foo).
Open it in Automator, edit the recipient address in the new message step, then after saving, copy it to your ~/Library/Services folder.
Here is the file: Send to Spam
To be less dangerous, I removed the message send and message deletion short-cuts at the end of the workflow. To re-instate them, simply run a “follow-me” for Shift+Command+D to send, and another for Command+Delete to delete the selected message.