> The spammer is a volume guy and is cheap. He wants to spew as many copies
> of his ad as possible at the lowest possible cost.
I suspect that this, like many generalisations, is not always true. I suspect, for instance, that some companies would see the cost of buying a subscriber account as cheap compared to the cost of getting an ad placed legitimately on LWN. There are plenty of dumb comment spammers that are already blocked by the site mechanics, others that are smart enough to bypass those but get blocked by general moderation, and fewer still that are prepared to invest the time (and perhaps the money) for what they see as advertising to a select, high-reputation community.
In this regard I think LWN is already filtering out the vast majority of unwanted comments.
Sadly, it sounds like these people aren't selling a service that is in LWN's sphere of interest, so even regular advertising isn't an option to them...
FWIW I'm in favour of greylisting new accounts, and perhaps giving meta-moderator status to subscribers of high standing. A 'web of trust' element might work as well - put in your GPG key ID and if it's been signed by one of the editors (tested by decrypting a URL in a message from them) then you're in :-) (for example).
P.S. A friend runs a website that allows people to post comments, and if you don't have the cookie you have to supply the name of a particular celebrity known in that community. Maybe asking new posters for the surname of the inventor of Linux would be a good filter to check the knowledge credentials of the poster... :-)