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Sure there is, you start making the distinction between ads and regular content impossible. If all of the images on the site and all of the ads all map to something like www.sitename.com/resource/somekindofhash, then how would you tell programmatically whats an ad? You could even assign each visitor a mapping seed, so that caching still works. Blocking would have to be done on a user-by-user basis, turning this into a MAD-scenario, where some users just start blocking EVERYTHING that isn't textual or generated.

Simply serving the adds from the same domain as the content. Ad blockers are really simple currently.

After that I imagine the blockers could step it up and start looking for script names, or reporting on common scripts shared accross domains, by which point we are in an arms race :)

By making ads indistinguishable from content. Or serving content from ad servers.