Limiting the number of Bloodfury Harpies will not be enough. The only true cure for their disease would be to slay their leader, Bloodfury Ripper! She is all but a queen to those filthy creatures. Slay her and their numbers will surely diminish. Go back into the Charred Vale and you will find her along the western hills. She will not be easy to vanquish, but the reward will be great.