The most important thing about writing the actual complaint email is to be clear and polite. While it's natural to sometimes be annoyed about receiving spam email and to be angry about it, you need to remember that you are not talking to the spammers but rather the people who can potentially stop the spam. If your tone is confrontational they are less likely to act on your complaint, and it's neither necessary nor helpful. You want to simply explain what is happening and include the full headers you consulted when trying to locate the spammer so that they can see what is going on for themselves.

Once you've sent your complaint, you may or may not receive a response, and if you do not it does not necessarily mean that nothing is being done. Spammers by nature send a huge volume of email, and thus complaints will often come in with a similar volume. A service provider will often not be able to reply to each complaint on a given spam problem, though most make the effort to do so as much as possible.