It's Your Job to Say No
There's a common perception that the job of the sales person is to get to "Yes".
But often, a true sales professional - someone who isn't simply selling, but rather is helping their client solve a problem - must be willing to say "No".
That approach won't get you to your end goal because...
What you're asking for isn't optimal for these reasons...
That analysis is interesting but it won't solve the problem at hand...
That's not what we do.
That last one is the hardest of all. Turn down work? Surely, we can find a solution.
Yes, sometimes you can. But sometimes you can't. Sometimes, saying No is the best answer. It means the client won't be wasting their time and you won't be wasting yours. It means the client will think more of you because you were forthright enough to say No, when others would have just said Yes. It means they can trust you to put their interests above your own. To be a true partner.
This isn't meant to be flippant. By all means, suggest alternatives. Offer alternative paths. Encourage different avenues of discourse.
Sometimes, though, the answer is simply "No".