Sorry friend but let's face it you can have a full group of people who "cleared" be 10x worse that that one guy desperate for a clear who probably would have been your top dps if they joined as such.
What I'm trying to say is that even with content in raid finder, there's no real chance of that group being legit even with a whole team of "farmers". it really makes no difference in the end to be honest. You either get a good group or a poo poo storm. Blocking people who haven't cleared with rf BARELY mitigates this.