The problem with that notion is how do you tell the difference between a bad guy pulling your feed and a good guy pulling your feed? I mean, you wouldn't want your normal users to suddenly get a bunch of cursing and such.
The short answer is that it is not possible. If you publish a site of any sort, then unscrupulous people are going to steal your content. Just a fact of life.
The usual answer to this problem is to make your feed content always contain a link back to your original post. Then the bot posts the link back to you as well, so any readers know where it came from.
Tell Google about these blogs too: http://www.google.com/dmca.html , so they can put in ignore rules for them.
Some products claim to detect these scrapers as well, I have not used them and cannot comment on them: http://copydefender.com/