I've actually had a similar struggle with the incessant but spammy recruitment emails, that I was able to fix almost perfectly. In the "Summary" section of my LinkedIn[1], I have a sentence that says this:

  "If you wish to contact me, please put the word 'Waffles' 
  in your message. It sounds silly, but it lets me know that 
  you have taken a little time to asses my profile, rather than 
  mass messaging people based on a keyword search."

"Waffles" will never be added by coincidence, so if I ever see that in the subject line of an email, I know that the recruiter has actually read my profile, and took the time to write a custom email to me. If it doesn't have the word "Waffles", I just ignore/delete it. It has dramatically improved the quality of emails I get from this vector

[1] http://www.linkedin.com/pub/joe-cortopassi/24/76b/5b9/