Adding hurdles will definitely reduce the number of applicants, but what you'll find is that if you ask for X, Y, Z in the ad most people will still send you whatever they were going to send you before (most likely a resume and coverletter). You then know they didn't read the ad and thus should probably be deleted.

This is why lots of job ads on craigslist say: "write XYZ in the email title" buried in the text to filter for those people who aren't even reading the ad.