I am fairly sure MobileMe (and possibly Mail.App) had a feature which allowed you to send a non-delivery notification for an email. This is possibly the best way to battle spam, however I can not find this feature anywhere now. Has it been removed or is it hidden somewhere else?
This page suggests that the feature exists in Mail under Lion. The feature, however, does not appear to exist, either as a toolbar option, nor as a Message menu command, nor as a context menu command, nor as an item in the Help file for Mail. The aforelinked document says "Lion" in its title, but refers to Mail 4.0; the Mail program under Lion is Mail 5.1.
This may or may not have anything to do with Apple's acknowledgement that bounce does not always dissuade spammers.