However, all of these can be forged, albeit with varying degrees of difficulty, Message-ID being the hardest of the three. The ultimate authority is the Received header. These lines document every machine that the email passes through. In fact, it probably came from neither. Every responsible site will have that address.
Many people send a copy of the complaint to the offender as well.
Include full headers from the message, and describe why you think it came from the site in question. Point out that you did not request this email, and make it clear you don't want any more of it. That is grounds for your own account to be yanked, and it will not help your complaint. The fundamental principle in fighting spam is to cut the connectivity. If the spammer is the ISP, the next step is to contact the company who provides a connection to the ISP itself.