I admit that I told that guy to do that since he seemed to be for real. In retrospect I should have just told him to put a whitespace somewhere in his URL. Of course, spambots will catch on to that idea pretty quickly too.
That said, in this case the poster should have just uploaded the schematic instead of linking to it.
If they haven't already then the spammers are either morons or not paying attention. Personally I don't think that the 3-post vetting period is a good idea. Technological solutions to sociological problems usually don't work, or at least not for very long. Even CAPTCHA was broken quite some time ago and can no longer be considered effective in the wild.