I'm going to take a shot at: yes.

We tried this approach too, and it worked great. But for every correct application we got two or three wrong ones.

We emailed them explaining the problem, to give them a chance, and only one replied - to tell me I was being unfair not to consider him....

We did, and since one of the unwritten job requirements was "able to follow clear instructions", it worked well as a filter.

Also, we posted the ad using one particular email address (a working, but incorrect one), but to apply to the correct address they had to solve an easy puzzle which consisted of looking at the page source for the ad, seeing that our logo image was being loaded from ourdomain.com/job/logo.jpg, and going to ourdomain.com/job for the correct email address to apply to.

I didn't want anything too difficult or annoying, but on the other hand, I knew anyone that could figure that out and follow the directions would work out fine for us.

Any chance of seeing a copy of the ad?