Mark Pilgrim goes into considerable detail about what I was recently discussing here on referer spam and similar topics. His basic point was that LoJack deters crime overall, but won't stop your car from getting stolen, whereas the club will stop your car from being stolen because the thief will go after another car...until such time that The Club is so ubiquitous that it will be worth the thief's effort to steal cars that use it.
What Mark failed to do in his discussion is provide an example of a LoJack type system in the world of fighting spam. I'd say the answer to that question is to make spam and all it's variants very illegal with extremely stiff penalties, far beyond what you would normally expect such a crime to carry due to the number of people affected and for how destructive it is to the community as a whole.
P.S. using the PayPal style system where they say "please enter the code contained in this blurry image with no ALT text" is a problem that spammers have solved by shipping out those gifs to humans to decode them and continuing with the automated process!
