A typical posting from a troll or very ignorant newbie might contain this:
>Your newsgroup?  Strange, I thought the Usenet was a public system.
>At least it was the last time I checked.
The correct response is:

If you "checked", you checked with an ignorant person or the wrong source.

USENET is not a 'public system'. USENET is an anarchy. It is a system of cooperating public and private systems. Note the word cooperating. -NO- USENET node is -obliged- to carry -anything-, except as it is under a specific contract to do so.

Under the newsadmin-to-newsadmin agreements underlying USENET, the custom is to complain about off-topic postings, and expect something to be done about them. To survive in a functioning anarchy, one must be very very polite, because in an anarchy, you don't have 'rights', just customs.