There must be several steps to go through before there is any heavy action taken against the site owner. You would have to have the chance to take care of the matter yourself, so to that end I would think getting the IP address and the ISP of the spammer would be the first step...along with the date and probably the time the e-mail was sent.

Then you contact the ISP with the information and the complaint and hope they can do something.

It would also help to record IP addresses of people who sign up for the affiliate program, just in case.