Christopher Hawkins is spot-on...

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…with his descriptions of types of customers (link from Avonelle). Read the comments, too  I have seen many of these types often in my seven+ years of real-world IT. I will add the caveat that I have seen most of these in the earlier part of my career when my employers were much smaller and did not have the discipline and processes in place to handle these situations.  I can only imagine what it is like being an independant and dealing with this. Thoughts on a few of these:

• The Bully – I actually had a member of the customer’s IT staff call me at 02:00 on my mobile (I was soundly sleeping at the hotel) SCREAMING at me with plenty of choice expletives that I had personally screwed up his computer (I had not – he had caused his own pain through lack of paying attention).  This individual always seemed quite tense (at the breaking point, actually) and would not hesitate to curse at you.  So much for being an adult and a professional; no one needs to behave like this.
  
• The Something-For-Nothing –  I can understanding giving a little away, but three words come to mind: “Scope Change Agreement”.  No signature, no work.
  
• The Flake – can screw you royally. Been there.  These are tough and can wreak havoc on your schedule and your sanity.  One option is to stop work until they catch up / deliver.
  
• The Slow Pay – One word: “lawyer”.  Although, depending on the customer, a dash of understanding may appropriate here (e.g. they habitually pay ten to twelve days late because their accounting system is b0rked).
  
• The Liar – Apply liberal doses of CYA and run away from this one. Run!
  
• The Money Pit – My experiences with these types was largely due to my employer being desperate for work and having the inability to say “no”.  Again, while I understand wanting to please the customer, there are limits. There is a difference between being generous to a customer and letting yourself be taken advantage of.

One commenter writes:

Who the hell are you to fire your clients? You work for them, they don't work for you, and they don't owe you a living. You sound like the typical whiney [sic]softeware developer who gets upset if a client dares to [sic]questioin your brilliance.

I think that, to directly answer the question, he is a service provider who desires to no longer be abused and taken advantage. I think that the customer the right to fire the service provider and the service provider has every right to fire the customer; working for someone is not exclusive of professionalism (on either end).   My $0.02. . .

-Nino