Well this is something I run into all the time. I have a favorite saying I've been known to use with clients frequently, "we aren't designing this site so you can buy something from yourself but so that other people will buy something from you and the site has to be designed within industry standards in that regard not simply to please what you want it to look like." Then I point to all of the certificates on the wall including the CIW Certified Professional Webmaster one. I explain to them that if they do not want to listen to the certified professional that has developed hundreds of websites that are successful and making money, then my service is not the one they want. Yeah its harsh, but it works. You have to position yourself and your authority right from the outset as if you are saying "its my way or the highway" otherwise you'll have nothing but problems from the beginning. When a client sees you this way they have more respect I think and as well trust you must really know what you are doing to be so bold that you don't care. At least, it works well for me (to a point) but then, a large part of my business is from the referrals of happy clients so that helps a lot when you talk to a new customer so forcefully and yuo let them know you are too busy and too popular to take any bull from anyone.
Of course, for this to work and work well you better be able to back up what you say and do otherwise it will certainly backfire on you.
So in your case, with reference to your post simonm, it might be time to say "Hey if you aren't going to listen to me and let me lead the process with regard to developing your site so it will rank well in the major search engines, then good luck to you because design alone is not the answer when you consider about 80% of your traffic will come from being ranked highly in the search engines and for that you are going to not only need the professional and qualified advice of a seo professional but actually follow the advice."

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