Off hand, it seems to me that CL should offer a method to report/inquire about lack of response to an e-mail. CL has the e-mail address of the listing party, and the messages from CL always make it through to the in-box when you're placing an ad, so they would be able to contact the seller and say something like "Hey, we're getting complaints that you're not responding, and in order to maintain the integrity of our site (insert eye roll here), we may have to pull your ad." That might get the seller to either delete the ad or check their spam folder.
Also, whenever I respond to an ad I do my best to include some sort of local reference that a spam-bot wouldn't know about, such as a landmark or nearby highway/exit that indicates I'm familiar with the area, and I'll usually change-up the heading slightly, which is something else a spam-bot doesn't do. Alas, while that might get me a response, it doesn't always result in follow-through. I think it has a lot to do with timing. Whoever catches the seller when they're inclined to make arrangements, wins.
I think we also might need to consider the source. These are likely the same types of people who don't bother to pull down the signs they plastered all over the place after their garage sales are over with.