...I don't know how you manage to do it, but you've got until 10am on Monday to stop using my domain name to stop spamming people or I'll report you to your ISP.
That's not a threat, it's a promise!
I know how to read headers and I've got all the time in the world to do it, so you've got 2 days to stop or I'll report you to your ISP as soon as I see you've sent another spam email.
Up to you! *shrug*