Take at look at the HTML source of a page when viewed with Readability - it's unaltered. This is all client-side post-processing. So the request to you is the same. In other words, it's only changing the display, and it's doing so without sending any info to your server.
You could try to code around it, but really, is it worth the effort? Like for example, since it does its magic by parsing the DOM, you could serve up malformed documents that can't be parsed, but that seems like the worst of all possible solutions.