My experience having done several client management studies with clients, is that jerks have friends ... who are also jerks. (Generalising, of course, but if you've got a solid enough business model you won't mind losing a few good clients if it means not dealing with a larger number of toxic clients.)

Best case study I did was with a branding agency. We got a list of their clients (about 100) and had the owner mark them each as Up, Down, or Neutral in terms of energy. We then had the team explore where each of the 'Down' clients (about 15) came from - half were from the Yellow Pages (again, using my experience, more price-oriented than service or quality) ... and the other half were referrals from the clients who came through the Yellow Pages!