Thanks to a small team and a lot of publicity, SEOmoz turns away a significant amount of work - sending folks to specific references or our recommended list or, occassionally, recommending they hire in-house SEO help. We're lucky in that we get to picky about the folks we choose to work with - some of the reasons for turning away business can include:

Obvious factors like:

• Size of the company/project/budget
• Industry focus
• Competitiveness of Niche Compared to Potential Profitability
• Conflicts with existing or past contracts/non-competes

And some less mainstream ones:

• Quality of Linkbait Opportunities (since this has become a primary technique for us, we tend to choose clients that can use it effectively)
• Ideological Affiliations (politics, beliefs, associations, etc.)
• Background of the Company or Principal Executives (this most often applies to folks seeking reputation management campaigns)
• Personality match or conflict with the primary contacts

I wondered if other companies do this too, so I've put together a quick poll. If you provide search marketing services (or business services of any kind), I'd love to get your (anonymous) feedback. Note that I removed a few from the above as I felt everyone turns away business based on size/price and competitiveness vs. potential (note that multiple choices are allowed):

I'd love to discuss this more as its a fascinating topic to me - client selection seems to be both an art and a science, and I'm not sure if it's something we've always done a good job with... How about you?