I made that offer because it seemd a good opportunity to test this scheme in a group that gets a great deal of unwanted traffic. And it is not just the spammers who can change their approach. I think the difficulty is overstated. And a human trying to keep track of keywords in a large number of newsgroups won't do it for free. If nothing else, it will raise the cost of available spamming software, and thereby disqualify a certain fraction of potential spammers.
Diversity is the key to stability.
You lost me on that last statement. Both need to make the required keyword public.