I can't believe this hasn't been mentioned before....Why don't you require that a geographic location be a part of the subject line before accepting a post???  It's a little annoying to have to click on & read an ad (which usually leads to a website..some of which take a while to load) only to find out that due to the location it's all been an exercise in how to waste time.

It would be extremely easy to do & couldn't hurt a damn thing.