He needs to read this
Usenet FAQ's as he obviously has a huge misunderstanding about Usenet. There should be other faq's on there to bring him up to speed.
He and his friends are harrassing
you - admittedly through ignorance of the situation - and if they look at the TOS of their ISP's they will see that they are committing "internet abuse". ISP's take that very seriously.
If having apraised them of the situation re Usenet, you were to let it be known that you might report
them to the abuse department of their ISP's they could lose their internet accounts for their behaviour. You'll be able to find an abuse address in the headers of their posts. You'll probably need to use a proper newsreader for this, as I don't think these scripts show them. I use Gilby's for my board, but I don't think they include that info. (I tend to read the newsgroups with a newreader mainly.)
What it boils down to is that
you are not committing any 'abuse' but
they are. (I assume you don't spam them, NG's hate that and that's abuse). If your board is just acting as a client and your members are reading, and posting to the NGs you have nothing at all to worry about.
So yes, I'd make them aware of the facts re Usenet and point out that
they are the ones committing internet abuse in breach of their ISP's TOS. That might hopefully put a stop to it.
Good luck.
Phoenix