I always like to piss them off by asking why they should get a cut of my hourly rate, since the don't know what they are talking about, how can they possibly add value?
Or another fun thing to do: send them your CV in PDF format, so they can't "improve" your CV for their clients, that usually pisses them off. I once had a "Professional international recruitment bureau" with "more then 40 years of combined experience" that told me they did not know how to open a PDF file...