As with email spam, you must use your best judgment. By normal usenet definitions, spam is a message which is either cross-posted excessively or multi-posted excessively. However, other forms of postings are often frowned upon by usenet administrators and ISPs in general even if not strictly defined as spam.
Usenet is much more complicated than email to evaluate. Each group has its own rules (for instance, most groups - but not all - forbid commercial postings). When reporting a usenet message, you should always add some comments of your own to indicate why you are reporting the message as spam. Indicate what local rules the poster is breaking and include a link to the group's charter, if available. Or, indicate that the message has been excessively multi-posted or cross-posted.
Under no circumstances should you report messages which are freedom of speech issues. For instance, if you are reading a pro-choice newsgroup, and you see an objectionable post from a pro-lifer, this should *not* be reported to the ISP using SpamCop or any other method. Simply being off-topic does not make a message spam. Reporting such messages will gain nothing and will only serve to dilute the opinion of administrators for you personally and SpamCop generally. See also the rules FAQ for possible punishment (including banning from SpamCop).
If you object to a certain post, you should also make every effort to educate the poster before you report them to their ISP. Usenet is a place where many people learn about netiquette for the first time. If you think the poster is doing the wrong thing out of ignorance, please try to educate them nicely via email before you cry spam. Don't clutter the newsgroup with these educational messages. Send the poster email. If the person continues to post spam to your group and they are unwilling to be educated, only then should you bring the issue to the attention of their ISP.
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