I wish there was a hiring site (cough Linkedin cough) that limited the number of jobs someone could apply to; or otherwise allowed hiring managers to say something like "I'm only interested in candidates who've applied to no more than 10 jobs in the past 30 days".

IE, I'd really like to push the effort on filtering back onto the candidate.

Even more, perhaps give candidates some kind of reputation based on how relevant / irrelevant / spammy their applications are. This way candidates who send out less applications and choose more carefully are given a boost.