I have been inspired over the years by the writings of a marketing guru, Seth Godin.  He is highly sought after as a speaker, very seldom gives speeches and is the author of 17 books.  Many people have the approach that the customer is king and Seth says that that is true except for difficult clients.  He says that successful organisations fire the 1% of the problem people that cause 95% of the pain.  He does say that we should try and avoid dealing with those people in the first place and find some or other polite way to decline taking on their work, but although the column I am going to refer you to is headed “The customer is always right”, he goes on to make it clear that if they are not right then they should not be your customer!  You can read the blog entry here: http://sethgodin.typepad.com/seths_blog/2006/04/the_customer_is.html.  

In another blog article he says that businesses get to choose their clients, and not the other way around.  He says you choose them with your pricing, your content, your promotion, your outreach and your product line.  He says that when you choose your clients you should consider a number of factors including how demanding that client is.  He finishes his blog by saying, “It is not a matter of who can benefit from what you sell.  It is about choosing the customers you’d like to have.”  I learnt many years ago that there are some people that you simply cannot satisfy, whether they cause trouble from the point of view of trying to barter or negotiate your fees, which is something I refuse to do, or they are just genuinely miserable, or they complain to everybody about everything and prevent you from spending as much time on the 99% of clients who have decent cases and are completely honest, reasonable and polite.  I have never been so desperate for business that I have been prepared to put up with dishonesty, abuse or absolutely impossible people and I have generally referred those types of clients to my competitors.  I think some smaller businesses and competitors with much less experience often make the mistake, in a desperate quest to earn a little bit more money, of ignoring this very sensible advice.