Of course I'm not going to say "no, stop being a moron, you actually want the seat up." I'll explain to them what actually make sense, say "let's try it my way and if you don't like it we can try yours." Then I'll set them up properly, they'll realize I was right (usually) and go away happy and a little more educated.As for the fit problem, same goes. Ask them why they want the adjustment, explain to them the options "I can do as you say, which will make it worse, or I can adjust X and Y, then Z and Q will happen, and you'll be happier. If said idiot understands what can actually make a difference, they'll be happier. If they're just too stupid and ignorant, let them ride with the saddle in the wrong position and they can suck it up. It doesn't really matter, so long as they leave your shop happy.
And stepping back to the douche with the derailleur "problem" I never said he was an idiot or making things up. All I told him was that, in it's current state, nothing was wrong with the bike, and offered up the only explanation I could think of for why he may have had an issue that resolved itself without any intervention. Might he have been happier if I'd lied to him, said I'd done something wrong, and pretended to have fixed it? Probably. Am I interested in lying to customers and admitting fault when I had none? No. Am I interested in falling over myself to assuage the complaints of someone who's going to be a whiney, angry, impolite cvnt? Nope. Frankly the shop's doing well enough that we can stand to have him go elsewhere, and as far as I'm concerned, good riddance. I understand that in order to keep a customer base we need to keep them happy, but I'm of the opinion (and my boss has repeatedly validated his support of this policy) that we've got no problem letting the one guy in a thousand who's going to genuinely be a dick to us walk away unsatisfied.