I know that it seems counter intuitive to turn business down, but some clients might be too "expensive" for you in terms of (unpaid) time and effort you will spend attempting to satisfy their demands and damage to your reputation after they badmouth you to everyone they know because you "sold shoddy equipment", "didn't stand behind your product", "couldn't solve the 'problem' ", ...
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We once took on an existing system that "wasn't working right". We were recommended to the client by a contractor working on the site. This was a high end contractor with whom we had a good working relationship and previous successful projects. (In the end I think that the contractor wished that he had turned down the project) The customer was a pharmaceutical heir with a staff of five full time employees running his house. Availability of funds was not an issue. Unfortunately, attitude and willingness to spend funds turned out to be major problems.

The installed system was difficult for him to operate. There were multiple rooms and, in our opinion, changing a couple pieces and installing a universal RF based remote could have solved his 'problems.' The fundamental issue was that he would not point the remote in the proper direction or hold it stationary until the macro commands finished spewing their IR. "I shouldn't need to do that" was his retort to any suggestions that we made. He was correct about "I shouldn't need to do that", but to get there he would need to spend some money, but "I bought good equipment and it should work -- make it work". We went around in circles until he fired us.

We have a good reputation in the area with both customers and dealers. I know that he mounted a major bad mouthing campaign about us because over the following weeks three different dealers contacted us regarding this project. All of us had exactly the same conclusion (Operator issues, won't spend money or change habits). Two of the dealers walked away, the third attempted to fix things, but was not being paid for equipment delivered.

I can't imagine how much business we lost because he bad mouthed us to his wealthy friends.

On the first cut it may seem counter intuitive to walk away from any potential customer. The point that I'm making is that, if one can identify this sort of potential (bad) client in advance, it is long term more profitable for your business if you walk away from the job before becoming involved. Do this gently, perhaps saying that "we don't carry the equipment that you need", and recommend that he work with another dealer (usually a dealer that we don't like). I can remember one potential client that I gently turned away, but told him where to go and what to buy. About two weeks later he brought a friend to the store, telling the friend to "buy your system here." (and he did) Over the years I've had several referrals from the customer that I turned away.