No, there doesn't seem to be spam filtering. I think this may be in
large part due to the fact that this is a Usenet group. I'd be in
favor of transitioning to a mailing list (which can be filtered) or
even a Google group. I suspect—given how little traffic this list sees—
not much community activity occurs around Rhino.
I think it's Usenet proper, not just an NNTP server hosted by Mozilla.